Wow, that's a nice looking engine and clean frame off resto! I'm afraid my bus doesn't look that swanky, but 30 years ago I had a yard full of Cadillacs and that kind of SBC power would have cost me ten grand in 1995 dollars. So I put this Cadillac in out of Dad's car.
One thing I finally did do, that I should have done before, is move the whole engine/trans assembly forward two inches. There were two holes in the frame, one on each side in front of the bellhousing mount, exactly 2 1/16" ahead of the old ones, so I just removed all the old rivets and, with front motor mounts loose, disconnected the driveline and just pushed each side of the assembly in turn, forward 2" til the holes lined up. Then measured, it all looked straight, so I drilled and bolted everything up. What this does, is give you enough room between the firewall and valve covers, to actually get to the bolts to remove 'em if you need to. I had to have the front segment of driveshaft lengthened two inches, and I was already reworking the exhaust so that didn't matter much.
I had to move/rework/reweld engine mounts, but I had already done a lot of that work back in the '90s so it wasn't too terrible. Had to move right side bracket forward so the new block hugger headers would clear; added benefit of no longer having that hot header pipe right next to the starter.
My point of all this, is that you might want to consider moving the assembly forward an inch or two-it can make a huge difference in serviceability, as in my '65 C60 bus. the original SBC wasn't even much fun to service being so far back in there.
Edit: yes I subsequently replaced that ugly air cleaner!
