Someone else will probably chime in with more specifics as to the best gear choice with your drivetrain setup (I'm still learning) but I swapped a Dana 60 out of a 1971 C-20 into the back of my '54 3600 no problem.
The axle is a hair wider than the stock one, think i gained an inch? on each side but the wheels still fit inside the fenders (and I'm running radials). The only fab work was cutting off the old axle perches and welding new ones on to fit the '54 frame.
My Dana 60 has 4:10 gears which work fine for me. I think there are more options though if you wanted higher ones.
The other common swap is the GM 14 bolt rear axle, which from my research would be similar work wise to install, but they are pretty easy to find.
Hope that helps some,
Chooglin
