By my calcs, the Buick pistons will be about 0.063" below the deck at TDC using the 235 crank in a 216. The stroke difference is 0.188, and half of that is 0.094. The compression height of a 216 piston is ~2" measured as close as I can, and the compression height of the Buick pistons is 1.843", with a difference of 0.157".
One slight issue is the piston pin diameter. 216 and 235 use 0.866" pins, while the Buick pins are 0.875", a difference of 0.009, which can probably be taken care of with a light reaming of the small end of the rod, although that may not even be needed with the split small end on the 216 and 235 rods.
The Buick pistons have a slight dish (0.15" deep) on the crown, which would lower the compression ratio a bit, essentially increasing the combustion chamber volume. It'd take a bit more ciphering to figure the exact compression ratio.
