well this is a very tough call. I know that getting the bird down to that weight isn’t going to be easy at all. The race weight includes a cage also, correct? Trying to lose 515 lbs vs 400 lbs is almost a no brainer there. Some of the drag fbods who DO remove the dash and run lightweight windows are only down to 27-2800 race weight. This also includes the use of dinky 3.5 inch wide wheels in front and lightweight aluminum wheels out back. So as far as weight category i’d pick the Mustang.
I have driven both vehicles and for turning purposes i enjoyed the mustang as compared to the bird. Bird was very sloppy and boat like. Seating position isn’t the greatest either in the firebird. The mustang felt more planted to the ground and although it felt like i was sitting on a piece of plywood being able to see the front of the vehicle some helped. The after market support for the mustang would be better for this type of thing. Although the ability to use shocks/struts springs for the bird allows you to use strano’s recipe for turning. He seems to be doing damned good with his setup. now granted his is an LS1, i still believe it would work out. My firebird is lowered 2 inches and it feels way better than stock. If i am not mistaken the Mustang tends to oversteer while the Fbod has issues with pushing (understeer) you may not experience this though considering you could set it up properly.
Motor- i’d have to go with the firebird on this one. N/a 3.8’s are beasts. Rwd version does not have the intake problems posed on the fwd versions. i owned my 3.8 for roughly 80k. motor had about 145k on it and it then developed a slight knock on start-up. I had the 3.8 mustang for just over a year only problem i had with that was slight coolant leak. The bird never lost to the v6 stang.
I don’t think i can honestly pick a clear cut winner without actually driving both back to back. I mean i love Fbods and all but the weight of the stang makes me want to pick that.