I helped a couple of Comp Elim teams race the NHRA World Finals this last week-end and talked to a couple of tire guys about your Gas BBC FC chassis.
We agree if you are making under 800-900 HP, a 33x12x15 tire on a 12" wide wheel would be about right. If you are over 1000 hp then you could use your 15" wide rims but go to a GY 33x14.5x15 , that is what a Comp champion A/AP 27 "T" on a chassis close to a FC is running with his Gray Motorsport Pro Stock 1400 hp motor with a 3 speed. And he runs 6.60's at over 200 mph.
You posted a reply while I posted a response, so with 600 hp & a 5000 stall converter, you might try a 31x12x15 Goodyear [ weighs 26 lbs] that I run on my injected SBC alky burning Jrfueler, or the 33x12x15 GY I at 1'st suggested. And you might want to call A-1 converters 360-574-9966, to get your converter updated to your motor combo, so your motor will be allowed to run in the RPM range it wants to. I.E don't run a converter that stalls at 5000, when the motor/cam comes on at 6000.
If you would like more input, call me 805-444-4489, cell
Jon