I figure it's time I update this thread as it has been a bit.
My '81 SC has been out of the garage for a bit getting some upgrades to support a really big one. I'll update when I receive the parts. Regardless, that has allowed me to make some progress on the '72.
At the end of November my family was out of town for the weekend to pick up her new GX550 (happy wife, happy life), so I took the opportunity to drive up to the Bay Area to pick up some parts to go on my car with the coming swap - 935K3 front end, GTU rear quarters, and rockers. I'm sure I got a few stares on the way home!
My gas mileage actually improved by .5 mpg!
Anyway, I managed to have someone come pick up the 996 donor car a few weeks ago. He works for a well-known 911 resto-mod company and is going to put a Cup engine and suspension on the shell. Pretty cool. And last weekend a gentlemen who races in "24 Hours of Lemons" picked up the SBC which allowed me a bit of space to maneuver some things around in the garage.
So I'm nearly certain the engine fits. I didn't have time to lower the car to check for clearance, but the measurements are certainly promising. Interestingly the 996 mounts are only about 1" wider than the normal engine mounts on the '72 911. The problem, which I knew, would be the torsion tube. Fortunately several people here have provided great images which provides me a path forward.