The 915 heads with the original 400 pistons won't get you anywhere near 10-1 compression.
You probably wouldn't need to rebalance the assembly with a forged 383 crank because the piston weight isn't that much different. While the 400 piston is larger in diameter, the pin height is lower.
John, the counterweights are totally different from a 383 to the 400 cast cranks.
