I have a 428 that had a problem with the rear cap. I brought the block and a 455 rear cap to a machine shop and they were able to make the other rear cap work. They pulled the dowels in the block, ground the donor cap, located the donor cap, drilled/installed very slightly larger dowels in the block/cap and gave the rear journal a line hone to size the bore properly. So far so good on that engine. I do believe the thrust cap causes some problems when trying to use different caps than what came on the block. Adding different caps to a block is doable but it is expensive and really needs to be done by someone with experience with Pontiacs.
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