[BRENT in 10-uh-C]

Paul offers some good things to check but based on the package of parts, ...and the fact that you say this is the 4th time, I think the problem may be with the combination.

Generally speaking, with a high compression head and a centrifugal distributor, the possibility of detonation is VERY likely. Adding to the fact of a heavy C/W crankshaft and plenty of cam & fuel to be able to smoothly turn up the RPMs, I think it is very possible you are 'hammering out' the babbitt of the rods from detonation. (...this would also explain the piston skirt damage as the piston is trying to rock.) Once the rod-to-journal clearance opens from the excessively high pressure caused by detonation, this allows the rod to 'hammer' on the crankshaft some which as it does just allows it to hammer harder as the clearances grow until the babbit is gone.