The valves are different diameter, so those have to stay with the right block, and the connecting rods have to stay with the crankshaft, but the rest is somewhat interchangeable. One problem putting P218 parts into a used B43G block is that there is always some bore wear, with at least of bit of a ridge at the lowest point of the piston ring travel. With the longer stroke of the P218, the lower ring would be encountering that ridge with every revolution.
Also with the B43G block, you can't use the P218 starter, since it bolts to the block, and the B43G starter bolts to the oil pan.