I'm lookin at the same thing with my 64 C-10. It's not the gas mileage, it's the high engine speed on the freeway that bothers me...so I don't drive it fast...and I just about get run over every time I take it out on the highway.
I've been thinking about looking for a 70-72 C10 rear end which (so I'm told) would have a trailing arm/coil spring setup, and 5-lug axles. This would work nicely with the 74 front suspension that I'm planning to install.
I was talking to a guy at a hot rod cruise the other evening and he suggested that a GM B-body rear end would fit...like a Caprice or full-sized Buick of late 70s-mid 80s vintage. I'm still not sure about that, since the first thing he suggested was a 76 Monte Carlo. When I reminded him that they were 4-bar rear ends with no track rod, he kinda stammered and suggested the B-body rear end. So take it for what it's worth. Personally, I'd believe it when I see it.
Good luck, Limey,
