There are 2 ways to go.
1. Put crank, rods, blocks and heads on the U crankcase, use the U starter flywheel.
2. Put entire 403 in U, use 403 power unit flywheel and starter. May have to grind clearance for the starter in the transmission bell housing.
Either way creates problems with water return to radiator. (I solved this by using a diesel return inlet. Turned it upside down. But I was putting the 403 in a U diesel. May be hard to find, only 700 diesel Us were made.)
Another problem to overcome was the water bypass. I ran a tube up to the extra pipe plug in the U water manifold. I think some people just plug it off. If you want a temperature sensor in the return manifold you may have to be creative to make it fit under the hood.
The U radiator may prove to be marginal if you intend to work it. For a few minutes pulling it should be o.k. Mine could pull the bog in 4 th gear, but would soon boil the water. An extra thick core in the radiator helped some, but its still marginal.