4 speed conversion - what went wrong?

I'll go for the battery leads as well, particularly the negative cable. Thinks about it. Cable is likely connected to your engine block. Bellhousing bolts to the block and starter bolts to the bellhousing. Your high current electrical path is exactly the same route. Paint, rust, or dirt can interfere. Try redirecting your battery negative connection directly to the starter housing, or provide a good solid jumper to the engine block.