3500 is 7000 lbs per axle. My rig has a GVWR of 8650. That's good enough for me. How do you like them so far?
I run about 7000# over the rear axle at all times. I have 9600# GVWR on my E. I tend to run overloaded, because I know my cutaway is built the same as its DRW twin - Ford just derates the SRW for the tires & wheels used.
These tires (2 for the rear) were a huge improvement. I never realized how horribly a 10/E 16" tire squashes around when its maxed out. It feels like I added swaybar. Plus the tire doesn't squat under the load, so it stays rounder and the rig rolls easier.
As I said, I've come into the rail wheels a few times. If I could have had them for free, then I may have used them. The fuss with getting the offsets optimal ($150-200 per spacer, plus nuts & studs as necessary) and still only having a 6" wide wheel that is likely underrated - plus not having a spare... didn't seem very feasible.
FWIW, try and get the wheels (and any spacers) from the same type of truck. Then you know the offset will work.
Right now I can get some 19.5" DRW wheels in 8x170mm pattern - cheap!. Even there, the offset, bolt pattern, 6" width, and 2600# rating are turn-offs. for the reasons I mentioned above.
Another source... find out who does the tire service for UPS vans in your area. They ran 19.5"x 6.75" singles on some of their older GM vans. Sometimes they have surplus wheels. I have spoken to one person that got theirs this way. However, when I inquired they didn't have enough spares to be able to sell any off.