You are saying that he used a stick from a early 560, year 1979 or 1980, but back then the M117 560 engine was on the drawing board. I think the 560 (SEC, SE or Sl) wasnt sold until about 1986 or 1987.

That just cannot be true.

Maybe a 450 iron (?) block manual will bolt to a m119.......MAYBE
Maybe the flywheel too....Maybe

If not, go on customizing it all. In the end you probably have done the same amount of work, and you have a (very hard to find in the first place) 5 speed gearbox fitted to a V8.

I'd rather take a bigger step, and go for a 6speed from the beginning.