for slants,,back in 1968, that was the first year for the larger register cranks, '67 and prior were the small register, so you should be ok on that,,
concerning using a slant from an automatic with a manual,,
I have heard that there were some slant cranks that were not drilled for the tranny pilot bushing. I know for a fact that there were cranks intended for automatic tranny applications that had the pilot bushing hole drilled to depth, but the diameter was not honed and is under-size. I know because I have one. I used the Dakota bushing in that one.