head leaking oil

I have a 1992 Toyota pickup with a 3.0 (6 cylinder) engine. It blew a head gasket at 320,000 miles after they were originally replaced on a factory recall at 75,000 miles. They have both been replaced. Heads checked for cracks, smoothed and valves reworked. Twice after if was put back together the right head started leaking oil that runs back under the manifold into the V of the block. Can you tell what is going on here? we are perplexed as to what is going on.

Thanks,
John