...people FAR more qualified than I lurking hereabouts...BUTT here's my $.02:
1. If you're getting a spec-racer---talk to people who own and race them
2. Having said that---I could be done
BUTT.......
3. One old rule of thumb (or tires) I proved to myself again, while experimenting with tires: never have the tires too asymmetrical, front and rear. For instance, if your fronts measure 9 inches of real ("applied") tread wide, then the rears shouldn't be much more than 11" wide. The result of ignoring this is "plowing" or understeer. Butlers seem to love 10" wide in the front and 12" in the rear (this latter sounds, as I've said in other threads like a prison nightmare).
4. I was told by none other than Ron Butler that the Butlers can fit 14-16" wide at the rear---he recommended "at least" 275s on the front if one were to do so. Right now,, I've got 235s and 275s. What's great about (and not to be overlooked) with these "smaller" (for a Cobra) sizes is that they will "teach" you a lot! Try your four-wheel drifts and you'll be able to pull them off with throttle and wheel with ease.
5. Huge, wet parking lots are the learner's friends.....
6. I'm waiting 'til some tire manufacturer comes out with 15" 305s again. I may have to go to 16" to get 305s or even 315s. Someone told me to divide the mm(etric)tire-size by 25 and this yields the tire's width in inches. A 305 puts 12.2 inches of tread width.
7. The biggest "meats" I've driven with were on a friend's Chevy Camaro. He had 14.5 in.-wide in the back and 11.5" in the front. The car had ZERO body lean, gobs'o' horsepower and the Mickey Thompson's just WELDED the car to the road. It was unfortnate that the car had a slushomatic, but the owner just blew up manny-trannies with that rubber and 550+ hp.
8. Good luck. Find FFR spec racers, they'll tell you what to buy unincumbered by too much "theory"!