Let me clarify a little more. Neither of the boys will be rear facing by the time I actually own the Mustang so fitting a rear facing seat isn't a problem. I'm concerned about whether or not the larger seat, the Cosco Alpha Omega, is going to be too big and not allow for enough leg room behind the driver's seat for the kid that's in it.
I have plenty of time to test it out at the dealer myself and if worse comes to worse I can always buy smaller seats to replace them. I'd like to avoid the latter though because they are really nice seats and they weren't cheap if you know what I mean.
Thanks for the help so far. I still have one more seat to buy and we were going to buy something similar to those we have already. If someone can show me that they don't fit well and recommend something that does I'm all for it.