Just how quick were the 273 Commando's in the Quarter-Mile.

mph
According to that same #40 bulletin, your 400hp, 273 auto, has a w/p of 2900/400=7.25, and should go a minimum of 120 in the qtr @ about 11.1 sec; all with a race chassis. A stick car, in the late 70s would have gone 2 mph faster and possibly dip into the 10s.
A 400hp,NA, 273 would be very special indeed. That's about 1.46hp/cid, and it's running in; very hot street360 or supercharged territory.
Sweet!

So how about telling us a little more about the build? Ima gonna guess your cam has an ICA of 80* or better?