Haggis, you should have tags on the drivers door, or drivers door jam, that have the model and vehicle weights info.
Ken, if it originally came with a NP205, then it should be a M1028A1. (M1028A2 is the same with dually setup.)
My '86 M1028 has a NP208, delco alts, 3600# payload capacity, 9400#...