An 8R19.5" tire will have nearly the same outside diameter as a 7.50x17" tube type. The bias ply 17" will be about 34" O/D, while the 19.5" will be around 33.5". If you go with the 225/70R19.5s they will be shorter, in the 32" range. A 9R22.5" will be about 38" tall, which is just about the same as an 8.25 x 20" tube type. Your 7.00 x 20s will be a good bit shorter, though, probably in the 36"-37" range.
Another good replacement rear axle is a Dana 70 from a '70s era Dodge motor home. They have the same stud piloted 5 lug x 8" pattern as the old IHCs and can be fitted with a wide range of gear sets. This would eliminate the need for wheel adapters. Stu
