When I measured the distance between the cylinder cut outs I never torqued the cases together but I did squeeze them together in a vice so the measurements given will provide a fairly accurate idea of the differences. The 850 is .178" wider or .089" extra boring on each side required to fit 850 barrels.

I have 2 sets of 850 cases to compare and they are identical. I only have 1 set of 750 cases that appear to be late model ones as the timing side casting is solid where the distributor plate is located on earlier models. These are replacement cases without original stamped numbers so I can't get the actual date for them. Given the extra thickness of the 850 cases I'd be reluctant to machine out 750 cases to take the 850 barrels. I'd just pick up some 850 cases and go with them. The only negative would be if you wanted to keep a matching numbers bike.