I fitted a 35W H4 kit from Les with no probs really to an R100GS.
The 2 leads from the 'black box' (rectifier?) that is attached to the ballast were not long enough to route with the other leads thru the hole at the bottom front of the headlight, so I removed the rubber bung in the speedo cable hole and used that. Even so, the insulated part that covers these 2 could do with being longer.
I have the 'black box' cable tied to the LH steering head gusset in front of the tank, and the ballast behind the frame tube and horn below the tank.
The cable mentioned above that needs to be longer prevented me from mounting the ballast over the coils as suggested by Bumpmuncher.
Did not have to change anything inside the headlight case - the burner mounting uses the H4 bulb fitting and the plastic cover fits nicely over the burner, with its plug in bit outside the cover if you get my drift !!
The only extra connections are the live feed to the battery and the earth - its an easy job.
HTH
MIke