8 lug GM or Ford wheels will work I believe. Easy to find some used 16" steel wheels on Craigslist or Facebook .
8x6.5 is the bolt pattern for the trailers that use the hmmwv wheels. Dont get 99-up ford 8 lug which would be 8x170, 8x200 or 8x225.... They really need to stick to something. Actually E series vans used 8x6.5 because my 2006 has that bolt pattern. Ive graduated to medium duty recently but the ford 10 lug is still not "industry" standard.
I actually have a set of 4 8x6.5 aluminum off that van that I need to get rid of (didnt have the heart to scrap them)... I may have to test fit to see if the clearance for the drum works with those. I dont see why it wouldnt.
One point, I think if there were pneumatic casters you would have so much rolling resistance it would be very hard to push/pull unless you aired those tires up well past their max. Yes running over a zip tie will no longer bring you to a halt, but my 140lb self with a running start probably wouldnt be able to start it rolling on soft casters.