It might be worthwhile to investigate a swap to a complete rear axle assembly from a Japanese pickup truck- - - -including the brakes. The lug bolt pattern from several Nissan, Toyota, and Mazda pickups are the same as the 6 bolt Chevy lug bolt pattern, which will allow you to bolt the original wheels onto the newer rear axle. The track width should be very similar to an early Chavy axle. The Japanese rear end might have a more highway-friendly gear ratio, also. Then you'll only need to deal with a front axle conversion to hydraulic brakes.
If you want to keep the original torque tube type rear axle, it might be possible to simply fit the hydraulic brake backing plates and drums from the Japanese vehicle to the Chevy axle with some creative adapting.
Jerry
