Really no need to swap axles. A '65 will have a Spicer/Dana 44F up front, and a GM 12 bolt Truck rearend. New gears are readily available in most ratios from 3.08 up into the 4.88 ranges. A 3.55-3.73 would be very good, depending on what size tires you run. About the only reason I can see to swap the fron axle would be to get open knuckles (stronger design) and disk brakes. You'll want to look for a '71-'76 4x4 Chevy to get that axle, most '77-'87 GMs used a 10 bolt up front. Both would work, though.
