I just finished a set of wheels for my '71 Bonneville and used the Buchanan rims and spoke kits. I ordered a WM2X19 for the front and a WM3X18 for the rear. Buchanan supplied the SS spoke and nipple kits for the Conical hubs.
Prior to ordering I tried to find if any one was using wider or smaller diameter rims with the stock forks and swing arms but could not get any information that would lead me to believe that any of this would work.
I mounted the Avon AM26 Road Riders in 3.25X19 for the front and 4.00X18 for the rear. Something that I just noticed was that when I went to mount the front fender there is not a lot of room up there between the bosses on the forks for the fender mounting bolts.
The front tire measures about 3 3/4 inches wide at the widest point. The bosses are about 4 1/2" apart which only leaves about 3/4 " of room for a larger tire to fit. I also note that getting the bolts in place once the rim and wheel are mounted is a bit of a tight fit. I think it could get dicey if the rim were any wider. Of course if you didn't plan to mount a front fender using those bolts then all you would need is to be able to squeeze the tire pass the bosses.
In the rear I have about 3/8" between the chain and the tire. Of course you could squeak a little more room with a little more off set so maybe you might be able to find a larger size tire that would fit. However, you still would be able to use the WM 3 with a 4.50 of some brands which would not really justify going to a WM4. A WM4 rim would allow you to go up to a 5.0 tire but then I don't know if you would have clearance for the chain.
In the end I just gave up on trying to figure out whether I could get any real benefit from larger rims and tires. I know they would look cool but I really don't think I could ride hard enough to exceed the traction of the stock sizes so could not justify the risk of buying and then being stuck with tires and rims that would not fit.
Anyway, I'm pretty well pleased with this choice, If you decide to try larger sizes and have success let us know, it will be good info to have.