I don't have any GR650 Tempter parts to compare, but I did do some checking on the parts microfisch. Does your GR have rear Drum brakes? Any of th GS models I saw from 77- had rear disc. So that would mean a hydraulic Master cylender, hose and caliper would have to be adapted. Not impossible but quite a project. The drive sprocket hub (drum) on several GS's is the same part 64611-31003 as a GR's.
Take a look at Big Rich's two build threads in his sig. They are both GR's and maybe they may be able to, or already have answered your question.
Onto my build... I shopped around on e-bay and found some parts for the Zook, and have more on the way. Found a pair of OEM chrome gauge covers to replace the damaged back on the tach (so now I have a spare speedo cover) They wre listed for $19 but offered $10 and he accepted. SWEET!
A pack of 6 SuperTrapp discs for $16 from Summit Racing, I put all of them in with the 4 I had but wound up removing 1 for a total of 9 discs... woke her right up. Gave it a much better sound and no longer running rich.
Then yesterday my Front brake switch from TASmotorsports showed up. WOW I ordered that just the day before! Mathew at TAS has always treated me right and ya gotta love them Atomic Fireballs!
Four black arrow turn signals, a hooded Lucas tail light, and Superbike grips are on their way from Dime City. THANKS HERM!