Since you are using the Ural Transmission, the flywheel should bolt right up to the later engine and be no problem. The only problem I've heard about with this mod is getting the transmission and engine accurately aligned. Even a slight discrepancy will make the whole thing vibrate like a vibrator with St Vitus Dance which isn't good for either your Nuts or your Balls
. I believe there used to be a design for an interface plate to mate the two available although I've never seen it. Might be worth seeing if you can find it... save re-inventing the wheel.
Rob