You don't usually have to actually 'buy' to try anymore. Go to a Babies R Us and most of them will allow you to try car seats in your car to make sure it fits before you purchase it. You could check the car seat compatibility database to see what seats other people had luck with in mustangs. I did a search for Mustangs, and came up with mostly expired seats.
There are some car seats that fit will in most vehicles: Britax Marathon being the #1 b/c of the way the base is designed, it fits even small seats. It can be tall rear facing, but that's a necessary evil in a small car when the child sits behind the passenger seat the passenger will be seated almost all the way to the front. Remember though, that the seat doesn't have to be 45degrees reclined anymore once baby can sit up on his own and hold his head up well. It can sit more erect, not straight up, but less reclined. This gives more room to passengers in front of the seat. Not much in a stang, but more.
Here's things to think about when getting a convertible car seat.
Remember these rules:
1)the BEST seat is the one that fits your child, fits your car, and will be used correctly 100% of the time. (This is why convenience features DO make a difference and ARE worth the money! If its easy to use, you're more likely to use it correctly.
2)Children should stay rear facing AS LNOG AS POSSIBLE!!!! The 20lbs/1 year rule is outdated and provides a bare minimum for turning kids forward facing. The American Academy of Pediatrics and the National Highway Traffic Safety Administration both recommend keeping kids rear facing as long as possible, up to the limits of their seat, preferably until at least 2 years of age. For good reason: A forward-facing child under 2 years old is 4 times more likely to be killed or seriously injured in a crash than a rear-facing child of the same age. A child's vertabrae do not fully fuse until 3-6 years old, before then, she is at great risk for internal decapitation. The spinal column can stretch up to 2 inches in a crash BUT the spinal cord can only stretch up to 1/4 inch before it snaps and baby is gone. In other countries, rear facing 2 - 3 - 4 year olds is standard, they understand that its safer. Here, we turn them as soon as we get to, seeing it as a right of passage thing or something. Ridiculous. Most convertible seats have a 30lbs rear facing limit, Cosco/Dorel/Safety1st/Eddie Bauer seats rear face to 35lbs, Britax rear faces to 33lbs.
3)Once you do turn them forward facing, they need to stay in a 5 point harness as long as possible. 4 years/40lbs is the minimum for riding in a booster, and most 4 year olds have no business using one yet. If they can't sit upright for an entire trip, they need the harness of a car seat still. And, even if they do sit properly, a 5 point harness is safer, so you want to keep them in one as long as possible. This is important to consider b/c most car seats only forward face to 40lbs. My son just turned 3, and is in the 95th% for height and weight - 40" tall and weighs 41lbs. He outgrew the 40lbs seats shortly after his 2nd birthday. It was a total waste of money. He now has a Britax Marathon, which goes to 65lbs, and will be able to fit it for some time yet. If I'd have bought it when he was born, I could have had one car seat this whole time instead of the 3 I wasted money on. They are more expensive for many reasons, this is one.

So...knowing all that, here's some about specific seats. Please don't get a Graco Comfort Sport. Car seat techs call it the 'crappysport'. LOL Its a crappy seat that won't last your child very long b/c it has a really short shell. The straps twist all the time, and if they're twisted they won't support your child in a wreck properly. They're hard to install. There's nothing good about these seats except perhaps they're cheap sometimes. Ditto the Safety1st 3-in-1/Cosco Alpha Omega/Eddie Bauer 3-in1 seats. These are all the same company - same seat, just different covers. They stink. Hard to install, b/c of narrow belt paths. Ever tried to wash a skinny cup by hand? Now imagine that skinny cup with pointy edges. That's what putting your hand through these to install them is like. AND they have too short a shell to really go to 40lbs. And they do not make good boosters. Also, most kids have to use boosters until they're at least 8 years old. Car seats expire 6 years after the date of manufacture, so no matter what they say it is NOT the last seat you'll ever need to buy.

The Britax Marathon/Boulevard/Decathalon (These 3 are the same seat, just different variances of luxury additions, with the Marathon being the cheapest base model) can be found for regular price of $269, and on sale at Albee Baby online for $209-$219. The Marathon a wide open easy to route belt path, which makes using it correctly a lot easier, as do the built in lockoffs, which mean you never again have to use a locking clip. I can get it into 4 different vehicles in under 2 minutes. Because of the way the base is made, it fits in most cars. And, it is worth the money, b/c it lasts usually twice as long as most other car seats (all car seats expire 6 years after the date of manufacture) lasting a baby till they are 5-6 years old, where most at Walmart will only last till 2-4 years old. Rear faces to 33lbs, then forward to 65lbs. and top slots 17inches tall, lasts most kids to ages 5-6 years old.

The Evenflo Triumph Advance (not the original Triumph, make sure it says Advance) is a great seat. $150 version at Babies R Us has padding similar to Britax seats, top slots of 17", harnesses to 35lbs rear facing, and 50lbs forward facing. $120 Walmart version just has little less plush padding. Wide open belt path, easy to install, though it doesn't have built in lockoffs. The harness adjusts at the front of the car seat, you don't have to take the car seat out of the car just to raise/lower the straps. It's one of only 2 seats that does this (The Britax Boulevard is the other, I believE), and it has infinite harness adjustment so the harness always fits perfectly until its outgrown. No more tugging straps to tighten them either. You tighten and loosen the harness using knobs on the side of the seat. As a major bonus, it can be used in a recline position even in forward facing mode. Awesome for kids who still sleep in the car. I LOVE THIS SEAT! LOL My son, who is too big for every other car seat at Walmart has the same amount of room in this as the Britax Marathon.

The Fisher Price Safe Voyage Deluxe Convertible seat is great, it is made by Britax, without all the frills. It's the same shell as a Marathon, but for less than half the price. $130 at Albee Baby online with free shipping, or at their eBay name, babysurplus. Rear faces to 33lbs, forward to 55lbs. Easy to use, fits in most cars.

These are the best you can buy that will give you the most bang for your buck. As a single parent I'm always watching the budget - I have no money to speak of, and these are what I would buy. (My son has a marathon and is soon getting a Britax Regent). They will last the longest. I only wish someone would've told me all this before I had him, I coulda saved some money for the extra seats I started out with!

All seats in the US must meet the same safety criteria to sell, so technically they all are safe, however, if its hard to install, then it won't be used correctly and that makes it unsafe. Also, they only must meet the guidelines, some seats barely pass, some exceed them. Britax exceeds the guidelines and is easy to use.