I would think that the main reason to use a 75/76 Gl1000 front end is to update to dual discs while still maintaining spoke wheels. The 77/78F models had comstar wheels which are a whole different look than spokes. The dual calipers on both of these settups are a trailing type (mounted behind the fork leg) that offers a good bit better performance and a bit less weight than simply adding another stock 750k (or f0/f1) type caliper and rotor. The GL swap gives you a factory engineered spoked dual disc setup with lightweight annodized aluminum (rather than chromed steel) DID rims, and stouter forks. For the 750 the F2/F3 front forks etc can be slipped into any years tripple trees although the F2/F3 trees mount the handlebar in a lower and farther forward stance and the distance forward/backward betweem the fork tubes and the steering stem is different (I believe shorter) reportedly improving handling so using the F2/F3 trees may be desireable. The Gl1000 front end will fit into the 750s headstock without modification but since the GLs forks are larger diameter and wider apart, you need to use the GLs trees. As to the 500/550s, I am not particularly experienced but there have been a couple of posts mentioning that the GL stem will not properly fit the 500 series headstock so some machine work is required. If the fork diameter and spacing is the same between the 500s and the 750s then the F2/F3 front forks etc should mount up OK to the 500s tree's. If not, then you are in need of machine work again. Hope this helps.
