The 750s to 76 in the Kmodel had more offset (frontwards). I think the 550 has more down offset than a 750 on top tree, meaning that your fork slightly shortens. Tubes will fit, but stem is probably longer on the 750, meaning a spacer somewhere.. should work with slight change in handling, and stock headlight ears cant be used. IF you are constantly hard cornering and worried about maximun handling dont do it.
Would I do it if it were mine?..most likely..