Re: Uh oh....This isn't cool....
White smoke is usually steam, indicating overheating or a coolant leak into cylinders. Get a good pressure test and pressure test the coolant system.
But before you do that, I would take a simple cranking compression test. Take all the plugs out, crack open the carb (or TB) then check cranking compression on each cylinder. They should all be within 5% or so of each other. If you have two bad cylinders (low compression readings) right next to each other, then good chance it's a blown head gasket. Hopefully, all cylinders read good. If so, pressure test the coolant system.
Also, since you say it's overheating, maybe try running it with no thermostat. Could possibly be simply a stuck thermostat.
IMO, the other likely culprit is a bad gasket, which pressure test will reveal. Did you have the block and heads checked for straightness?
Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
Bought my first big-block Chevy in 1970