Napa and Rock Auto only carry them up to a 3900 model. They are similar, but not interchangeable. The 1.5 and 2 ton trucks use a slightly longer wheel cylinder than the 1 ton and 1.25 ton trucks did. The shorter cylinder will actually bolt in place and 'work', but the adjustment range will be too short.
Rock Auto and Napa both carry the 1.5" rebuild kits, but the largest cylinders they show online in their catalog are 1.375" bore.
