Are 10 x 2 1/2 rear drums big enough?

Even the 11x3 cars had the 11x2.5 rears. The rears do about 30% of the stopping power and stopping from about 120 MPH with front discs and even 10" drums should be no problem. Rear discs are best left for road racing where you have repeated hard stops (the only advantage discs have over drums is they run cooler) but the drums are a better brake because of the servo action. I have 11x3's on my 66 Bel II and the original 10" in the rear and it's fine even on hard stops. Another thing, I have a 68 RR that came originally with the optional Bendix 4 piston disc brakes and it came with the 10" drums in the rear.