Without reviewing a parts diagram, I would say that your most likely option would be to use the /5 parts since the clutch is housed in the /5 flywheel. This assumes the flywheel is OEM.

I installed a 5 speed from a 1974, R90/6 in a 1973 R75/5 many years ago. In that case, the transmission fit in the allotted space on the /5 with no additional changes. You must use the throw out assembly and clutch pushrod from the 5 speed. Other than that, all the /5 clutch components were retained. 

I’ve only ever owned a /7, so I’m no expert on this - However, visiting the EME website, their “ClutchKit332” indicates that it fits pre-1980 bikes including /5’s, /6’s and /7’s. To me that seems correct, but again, I’m no expert. There are probably differences in the clutch throw-out bearings and all those related parts, but the clutch itself may be all the same. 

Thanks, Im thinking if it's just a renewed disk than all years are compatible. James and David from what I have read yesterday I think your correct re the throw out bearing.  The clutch pack proper should be same for all up till 81.  I will measure the plates and order what is needed - hopefully nothing.  Thanks for the inputs. 

No free play is an adjustment. I would try adjustment before replacing the clutch. The procedure is in the owners manual. Basically you adjust the cable to get the correct exposed lower cable length. The you adjust the free play with the clutch adjuster arm screw. TC write up the procedure. http://www.rubberchickenracinggarage.com/Downloads/Airhead%20Clutch%20Cable%20Replacement%20and%20Adjustment.pdf

Well here two years later and still not renewed the clutch.  Frank is correct about freeplay being an adjustment.  I was incorrect in my using that word above.  I alway's had ample freeplay adjusted.  What I should have explained is the clutch disengages with VERY little additional movement past the freeplay indicating a worn out disk.  To my knowledge the clutch has yet to slip but I don't accelerate hard up hills.  

My hesitation is simply who to send my transmission to for a refresh.  It's a smooth case non kickstarter five speed.

Any suggestions would be appreciated. 

Most likely, there are several competent airhead mechanics in each state, so it depends what state you live in, where to take it.

You have put two more years of happy, curvy roads on the clutch since you first posted and unless it starts to slip (or there is something else going on with your tranny) I would just ride it. Remember these are automotive style dry clutches that generally work fine until they start to slip or chatter (in my experience).

You said that a $50 used clutch was installed with the tranny, do you have any knowledge how much wear it may have had on it when it was installed?

It may have been a gently used clutch with only a few thousand miles on it from a wrecked bike when you acquired it...