I've run so many combinations on a 273 from stock 2 barrel, to stock Commando, to 284 duration Offy intake and 600 Holley, to my favorite 273 with the 340 heads and a 71 340 TQ. My 64 was a 4 speed with gears from 2.76 to 4.10 and everything in between. I used to power shift at 6,500 rpm, and I missed a few, scared the heck out of me watching that tach flash without a rev limiter. Never had a problem with head gaskets. My biggest problem was keeping a clutch to hold it.