On my 55 Chevy Gasser project, it came with an early 60's Chevy van axle. I had a hard time finding a disc brake setup for it. You may want to look at Speedway Motors for a complete straight axle unit. They come with leaf springs, and shackle setup. Also, you can get the spindle and rotors/disc brakes to fit from Speedway. Since you need to find a Gasser style straight axle, that might be the easiest way. You just get rid of the A-Arm stuff, measure the geometry and weld the hangers to the bottom of the frame. Then the spring perches get welded to the axle below. For steering you can use a crossover tie rod from the existing box, or get an omni rear steer type rack and pinion and mount it behind the axle. No bump steer that way.