hiya Russ, someone asked similar not long ago, and I'd had same issue with my rebuild. I think what I did was tighten the front (top and bottom)mounts roughly central in their holes, and then packed the top g'box lug with large SS washers to centre it and align the Sprockets, then packed up the bottom lugs the same. What else can be done without going berserk and going to the expense of welding and stuff when all I want to do is ride it
The top lugs on the few cases I have are a bit mashed up and vary in width, but being precise, the widest is kinda-sorta-roundabaout-roughly a bit over an inch- maybe like about ~1.104", or just under 28mm (not forgetting it's a bit mashed).
I did a bit of assuming that the top lug/frame interface is central to maybe the centre of the tranny-engine interface bolts
Bottom lugs are I think not much better, and guesswork involved- just packed 'em up and took it to the frame straightener, and it rides real good(well).