Dirk,
You did not say street manners or track manners.
My personal experience is that you want similar diameter tires so the car sits level, not too much rake. With 295/50x15 rears, I personally like 235/60x15 fronts. Close to the same diameter. You sort out handling with tire pressures, sway bar rates, spring rates, etc.
These are Goodyear Eagle GT II upon arrival:
Track tires same idea (see
Roger Kraus Cobra Tires):
26.5x10-15's rear are 26.7" diameter
and
26.5x8-15 fronts are 26.2" diameter.
Depending on you roadsters front/rear weight bias, anti-sway bars, spring rates, are rear suspension setup, tire pressures vary. On my FFR at 55% rear weight bias, IRS, sway bars, and 500/500 lb/inch springs front and rear, we have found that 24 psi front and 25 psi rear on Goodyear racing slicks provide superior grip in corners.
Each car is different so my experiences will differ from the next person's.
Good luck sorting it out.
Larry