Hmmm....3.55s or ???
My 70 ran a best of 15.3 when it had:
stock 318, with a 340 singleplane intake and a 600 CFM Holley (dual points dist.)
727 with a stock converter
2.76 open rear gears
This is a full weight car also (well, not now as it's missing it fenders), I would think you could get better with a 4 speed, maybe something else is up.