AW is spot on. Don't be tempted to fit 40mm bings instead of your 32mm carbs.
I was fortunate enough to own a GS imported from the States and a standard Euro GS at the same time. The 32mm equipped US spec machine was significantly faster than the euro bike. it pulled like a train up to 110mph and then simply wouldn't go any faster where the 40mm equipped bike didn't have as much midrange and would claw a few MPH past the 110 MPH mark. Since we don't ride airhead GS for their high performance ability I'd take the 32mm bike every time.
Gearboxes are identical on virtually all of the bikes it's the final drive (bevel box) that varies between the models as long as you buy a 1981 onwards box.
Don't sweat the circlip issue it isn't that big a deal.