I agree on getting your local muffler shop to make a wye for the exhaust.
Motor mounts. If you use the 216 bell housing, you can use the rear mounts for the 216. They can be purchased through your local parts store, and will need a little rubber trimmed. I assume you have the front mount under the timing cover, which you can use, but I think you need to get the rubber from one of the old part vendors.
Temp gauge. If you still have the tube style temp gauge, the hole in the 235 head is larger, an adapter is available from the vendors.
Throttle linkage is farther aft on the 235. If available, get the linkage with the engine. Otherwise you will have to modify the 216 linkage.
Fan & water pump. The 54 235 should have the high mount pump, which will work in your 53. The fan from the 53 will probably be too long.
Drive belts. The 216 used 3/4" belts, the 235 had 1/2" belts. You can easily swap the pulley on the generator to 1/2".
