On the GL1000 I like to lace the wheel up for a 15 inch automotive tire. You can run a 135 15 as used on Renault cars in the 1960's and 1970's A teh 1975 and 1976 GL1000 are gear high this also helps with the gearing as it is smaller diameter. The 15 inch rim must be sized for an automotive aplication not a motorcycle aplication as they are not the same diameter. As motorcycle rims and automotive rims have a different spoke count you will need a rim blank and have it drilled or make a new wheel from scratch.
Stroker who is on this site should be able to help you with a custom wheel. Other wise, buy cheap tires, they tend to be harder rubber and as such last longer and even if they do not, at least they are cheap.