With the 16hp off the frame, I loosely bolted up the clutch on the new 23hp and set it in place to get an idea of how everything lines up. I jockied it around until the pulley was aligned with the marks I made from the 16hp.
With the engine shifted forward a few inches thanks to the new clutch with the inboard pulley, I have a ton of room on the flywheel side. I'm thinking the SEW spacer I bought (JD318 Onan to Vanguard) will need about an inch taken off the length to be correct plus the aforementioned raised pilot and new bolt circle. The length was the big question. Tomorrow I'm going to pull the blower housing and debris screen off, mock it up again, and take an accurate measurement from the face of the pump adapter to the face of the flywheel.
With the engine in it's correct location, I tried closing the hood with no luck. I took the side inserts and the heat shield out and it almost made it. It has about an inch left to go before it hits the taller air cleaner lid. It looks like raising the hood at the hinge by about an inch will work.
The clutch anti-rotation bracket i just bought doesn't fit either. I used the one referenced earlier in the thread in a print for a 4023 application and it's intended for the old style clutch with slots in the 5 o'clock and 7 o'clock positions. My clutch looks like the 4023 that Bob shared a picture of with holes every 120-degrees. No problem though - have welder, will modify.