3/4 Ton rearend choices for 1966 GMC

[runningiron]

I'm thinking about buying a 1966 GMC 3/4 ton that has a rebuilt 250 inline six with a 4 speed granny gear, new clutch, pressure plate. The seller doesn't know the rearend gear ratio, but tells me it doesn't like freeway speeds. I would like to swap out the leaf spring full floater Dana 60 to a later model GM semi floater 14 bolt rearend with highway gears. Anyone have info on what year 14 bolt would fit in the leaf spring 1966 GMC? I'm hoping there might be certain years that are a bolt in. I also plan on installing a Clifford 4 barrel intake and headers and would like to run a 700R4 trans. Thanks, any info would be greatly appreciated