Well lets see. Taking it one thing at a time, custom wheels are probably out because that would also move the inside wheels closer to the frame, which isn't a good idea. Just using some type of longer wheel stud with a spacer would put a lot of stress on the studs.
I think IF someone were to come up with something to be able to run large tires in a dual setup, it would require an actual fabricated spacer assembly. What I'm picturing would be one of two things, depending on how much space is needed. For a small amount of space (maybe up to an inch and a half or so), use a machined steel plate, which bolts to the lugs after the inside wheel is installed. Then the plate has 6 studs installed on it (indexed offset from the 6 mounting holes) for the outer wheel to mount to. If you look on ebay for wheel adapters or wheel spacers, you will find many for passenger vehicles like this.
If you needed more room than this, I think a welded assembly consisting of an inside steel plate with 6 holes, a spacer ring, then an outer, thicker steel plate with 6 studs installed.
The downside of using spacers is that depending on how far the outer tire gets moved out, it can put more stress on the hubs and bearings. This isn't as much of an issue if the truck is built as a pleasure vehicle and won't be loaded to max. capacity.
Just a few thoughts.
Jim