How to correct the compression ratio.
You can mill the heads to 70cc (aprox a .040-.050" cut) and swap the head gasket for the GM 350 .026" head gasket. You must drill the 400 steam hole in the gasket yourself.
If you were to home port the heads while they are off the motor
it will change the whole nature of this motor by making all these corrections.
What is the 400 cylinder head casting number?
use champion RV8c or AC R42T spark plugs .035" gap.