Hi. Running thought on how the prop would fit if I was to replace the pinto with a bike engine.

As I'd need to make new engine mounts on the xmember, prop length came to mind. Rather than taking one to a shop to be extended/balanced etc could I not use a longer prop off a transit (need to google lengths)? That way I could bolt one end to the diff and engine and make mounts based on that length.

Or have I missed something?

The ends on the prop won't be right, and you would compromise the position of your engine to make it suit.