has any one torn into forks enough to know if they are all the same internally. can i put 907 lowers on 750 uppers?

no,
early 907 lowers are for 300 mm discs, late 907 lowers are for 320 mm discs and a 750 has 280 mm discs.
I'm not sure about the total lenght of the legs due to 16" on the 750 and 906 and the 17" wheels on the 907's

ducbertus

I think that the lowers were the same internally. The changes, as was pointed out, were for the caliper mounts. The upper fork legs are certainly different lengths between the models, and I would assume the damper rods that go along with them. I'm trying to remember the history of them, but it escapes me. I think 750 and 907 were the same length, and 906 was different to accommodate the new radiator, or something like that. do a search and I think you will find the answer about 2-3 years ago. The first year 851 had the same forks, but was the shortest overall, and shortest travel.

Cheers,