Looks like the decision has already been made, but my $0.02 worth:
We used 413 and 318 Chryslers, 292 Chevys, 350 Chevys 454 Chevys and 455 Olds on irrigation pumps.
I would never use a 318 Chrysler or a 455 Olds based on those experiences. They did not hold up as well as any of the others.
If you are working on a 455, one of the trouble spots always seemed to be the strange little 3-cornered spot where the intake manifold, the block and the head came together. Never could get that to seal worth a darn, always leaked water.
