I have seen some early bellhousings for the heavy duty 4 speed that had both patterns on them. It's the one you had to unbolt the flywheel to get to all the bellhousing bolts.
This is correct, do not conflate truck and car stuff, especially in that era. Flywheel is probably different though. The HD trans you mentioned is the NP435.
But if I recall correctly, and the last one I did was in the 90's so it's fuzzy, there is an adapter plate between the slant and the bellhousing?
I don't have a truck parts manual, but it should confirm the same bell for small block and /6. No idea what the big block trucks used though
