Another vote here for the 261 cu. in. stovebolt six. Chevrolet built these motors specifically for large truck use and the torque characteristics would suit your one and a half ton well. Use the Carter bellhousing that Paul mentioned and the SM420 is a bolt in proposition.
I still have the crash box behind my 261 and you'd be amazed at how smooth they can shift once you get the hang of rpm matching and double clutching, granted not for everyone but a lot off fun once they're mastered!
-Mike-
