I did a little more work with the transmissions.
I got the bell-housing off the Chevy T5. It was pretty easy, just four bolts from the outside, then pry. It came right off.
One thing I discovered is that the tip of the transmission shaft on BOTH the transmissions is 15mm. That means that I SHOULD be able to use a stock Mercedes pilot bearing in the crankshaft, instead of finding/making something custom.
The length of the shaft on both transmissions is pretty close too. The T5 is just a little over 7 inches. When I measured the depth of the Mercedes bell housing on the engine, it was just under 7 inches to the mating surface. I think that I just need a 3/8" or 1/2" adapter plate and I will be all set.
At this point, I'm leaning towards using the Mercedes bell housing on the T5 transmission, with an adapter plate between the two. The bell housing will fit stock on the engine without having to move the starter and will have perfect alignment. I won't be able to use the original slave cylinder position, because the T5 transmission is too big compared to the little Mercedes 4-speed. I'll need to cut into the bell somewhere on the left and add a slave cylinder there.
Also, I've been sick of staring at the 6 flywheel bolts that have been holding the flywheel to the crankshaft. The bolts used for the torque converter are LONGER than those used for a manual transmission flywheel. They can't be threaded all the way in without hitting the engine! So, for the moment, I used washers and oversized nuts as spacers, which worked fine to hold the flywheel on. A friend dropped off some diesel related stuff a while back, including an assortment of flywheel bolts. (I think they are off old VWs.) They were all the right thread pitch, and I found enough matching ones to hold the flywheel on. When I looked up the bolts, I was surprised that they are $8 each, and 12 are required. Yowza! I just saved myself $100 if I can use these other ones!
PS: Here's more about it on my blog:
SuperTruck: Bell-Housing