Sorry I didnt mention a year. I am looking at purchasing an 81 and I know that the front wheel rivets are a bit on the loose side. The owner is a large man and rides hard. I have never liked the idea of riveted wheels and prefer the cast. I may need to look at an 83 but those seem scarce. I really like the look of the earlier GWs so I am interested in that range of years. Is there any other option for wheels, even spoke?
I'm guessing as long as Honda didnt change the drive shaft final drive from the gl1100 to 1200 I could use a 1200 wheel in the back. In the front I have done fork swaps on many bikes so if all i need to make is some spacers for the wheel or for the caliper that would be fine. Also might investigate just doing a front end swap, just need to measure triple tree width and fork length. Of course any help is appreciated. Its always easier if someone else has already done it than to have to go thru 3 or4 sets for front ends.
The 80 and 81 are 19" front and 17" rear. The 82 and 83 are 18" front and 16" rear. The following comments apply to OEM parts swap.
To go to the later size wheel/tire the swingarm must be swapped. I also had to trim the bit on the rear fender that holds up the plastic inner fender. The wider tire rubbed at full suspension compression. The earlier final drive should be used as it is slightlylower (by .001) than the later final drive. Because of the smaller diameter wheel/tire the revs for a given speed will be up. Ideally you'd go for an engine/transmission change with this which would get the engine revolutions back down to where they should be.
The front appears to be a straight swap but when I went to do this myself it appears that there is a lip inside both sides of the fender that will need triming. The speedo will be off too unless you swap in a later speedo head.
GL1000s had spoked wheels, I have no idea whether they could be made to fit and what sizes they are. The 83 had solid (as in no rivets) and they should fit as discussed above. The brake parts will also swap over but can't be mixed.