I believe the pistons were designed differently for the 25 and 30hp powerheads, hence the two different part numbers. As far as I know, the location of the wrist pin was changed slightly to play a role in the port timing to give the 30hp those 5 more horses than a 25. But I have no proof of that, only what I’ve heard.
In theory, the 25 pistons should work, and not hit anything throughout the piston stroke. The unknown is what horsepower loss would occur using the 25hp pistons in a 30hp block. I’m pretty sure the block part numbers are different for 25 and 30hps but I would think that the worst that could happen using the 25 pistons would be that you make your 30 into a 25hp.
Measure and compare the dimensions of the porting and wrist pin location between the two powerheads you have, that should give you the answers you’re looking for
