So, sikyne, how did you figure out the lateral dimensions, to get the clutch throw so it would still contact the clutch plate correctly?
Did you cut a fake crankshaft or something to get the case lined up with the engine? I'm thinking if I did this, I'd cut something that was a tight slip fit on the transmission intake shaft, that then expanded to the bore size of the block, so they'd be aligned and perpendicular, but I still have to worry about distance between transmission and engine block and ending up with a slipping clutch.
I'm using an A-series engine in a totally different car, with no driveshaft tunnel, so that's less of a concern.

I turned a straight shaft to fit the main caps of an old block and extended it out the rear to locate into the bolt on snout of the bell housing. This kept things inline during preheating and welding. The length was just worked out so the modified 71B input shaft protruded the bell housing mounting face the same amount as the original 56A box.

The throw out bearing remains square to the pressure plate regardless of travel. I cant see any problems with clutch use/slipping. You could dummy it all up on an old engine before you weld the halves together if you had concerns.

Cool, thank you. I may give that a try.