A 3.55 won't kill you without overdrive. This is the combo in my 89 Ranger : Explorer 5.0, Toploader 4 speed, 3.73 rear 28" tall tire 60 mph is 2687, 70 is right at 3000. 3.73 with 28" tires is the equivalent of 3.55's with a 26" tire. (what you'll have in a Stang) I drive mine everyday.
289, T-5 Manual Trans, and 3.89 in a Detroit Locker ... as far as RPMs, couldnt tell you. I've never had a tach in the car, I just go by sound and feel. But I'll tell you what. 3.89's feel REAL GOOD, especially with a locker in a 9"
my 68 has a ford 9 inch with 35 spline axles, 3.89 gears and a detroit locker.
I am going to put a TKO600 in it and the engine is a 532 big block ford.