I have built wheel adapters for 3 different tractors, and while I live in "flat Florida", I have regretted doing it on all three.
Ford 7710 4wd was the first, measuring 13'3" across the rear. When the axle broke, it was expensive!
John Deere 2555 only stayed on for a few weeks, haven't had any axle issues.
Ford 1100 4wd subcompact. Cute as can be with 6 tires, but when an axle breaks, they are hard to find. I had actually sold the tractor to a friend and he only drove it across the yard before it broke. He called me and asked if I had any axle trouble with it, and I said no, before he told me what happened. I felt really bad, and did some research, pulled a couple favors and got a new one shipped in for 1/5 of what the local dealership quoted him. He was happy at that point. He was looking at what it was going to cost to build one from scratch and it wasn't cheap!! (he owned a machine shop capable of doing it)
The duals may or may not have had anything to do with the axles breaking, but I resist doing it nowadays, as the costs far outweigh the advantage!
David from Jax
I used heavy wall pipe with ends welded on them, with the bolt hole patterns drilled in one side, and the other side drilled and threaded.