The bell housing bolt pattern on the Chevy and GMC engines are the same for standard trans applications. I can't think of a reason the H-M equipped blocks would have a different bolt pattern. I believe the H-M equipped engines had eccentric dowel pins that located the adapter ring for the transmission, and the ring needed to be checked for runout with a dial indicator bolted to a fixture attached to the crankshaft hub. That was done at the factory, which was why an engine block and adapter ring needed to stay together. The same procedure can be done in the field, but only by someone who knows how to use a dial indicator to check runout when installing the adapter ring between the block and the transmission. The fluid coupling (NOT a torque converter, BTW) is very sensitive to alignment with the crankshaft.
Your buyer could contact FATSCO transmission parts in New Jersey for a lot more accurate information about the differences in Chevy and GMC H-M transmissions. They specialize in obsolete automatic trans parts and repair manuals.
Jerry
