A 3.8 won't bolt to the tranny like a 3.5? They're both m116's. I would think they're compatible. Hell, I'd imagine a 5.6 would bolt on, although it'd probably tear the tranny apart.

I would also think the manual could handle the 4.5. Sure, the 4.5 has a bit more torque, but unless the 3.5's tranny is marginal as it is, it shouldn't be a big deal right?

Probably common in Europe but not over here, it sounds tempting, but I'd pop a 4.5 in it if I were you. I'd try to put it on that manual, it should bolt right in. And although I've never had the blocks apart for comparison, I'd think a 4.5 (m117) block is slightly taller than a 3.5's to account for the longer stroke. it might only be an inch or so, I don't know. But I doubt MB would make it two diferent numbers if it were the same block, as they kept the m116 and m117 designations when they increased the bore in future versions.