NV3500 into 90 3/4 ton truck???

I have a real nice 90 250 4x4 with a 360 TBI and a 4speed granny low trans and it really needs another gear by about 35 mph. I also have a 97 1500 4x4 with a NV3500, is it a bolt in deal or do I need to change a bunch of junk?