8.25-10 tyres are certainly the correct tyres for the 6pdr, but I don't believe that there is any close replacement availble for them. (See my earlier thread on 9.00-13 tyres)7.50-10 NDCC tyres from a M274 Mule are made in the States, and while close in size, they are only 4 ply and not strong enough, at least not for road use, maybe OK for static display.
Heavier tyres are made for forklifts like these fitted to a Moffet Mounty, a small forklift that attaches to the rear of a truck chassis. These are the right width, overall diameter and ply rating and look OK too, but are in a 27-10/12 size which means they are on a 12 inch rim size. This means you will have to get new rims made up. Forklift tyres are made for 10 inch rims, but are usually low profile meaning your gunshield will be foul of the road surface.
