I too did some research on this topic a while back. I'll pass along what I know, Isuzu cabover comes in a GVW range from 12,000lb (NPR) to 19,500lb (NRR HD) There is a NQR and a regular NRR in between.
There is a large Isuzu salvage yard near me that let me walk around and measure to my hearts content. I measured an '01 NPR 12K GVW. I came up with 76 3/4" WMS to WMS, and 74" outside tires. The owner told me the axle itself interchanges between series, the difference being in the brakes. He said that the rotor on the NRR was thicker, and would probably add about and inch to the WMS dimension. He quoted me $600 for a complete axle, brakes, rims of my choice and sway bar.
Also the wheel bolt circle is listed as stud centered 6 on 8.75" with a 6.46 inner hole. Rims from an old Ferd F-600 are 6 on 8.75" so you should be able to match the rears at least in style by either opening up the center hole or using a shim ring. The largest rim that Isuzu offered in a 19.5" but I'm sure there are 22.5" out there. I've got some more measurements if you need them.
Scott
Salvage yard:
http://www.busbeetruckparts.com/
