dogdaddy - Your axle appears to be out of a mid to late 60's or early 70's GM A-Body passenger car (most likely a Chevelle). A common swap used by many folks. However, the PO didn't do you any favors by using this setup in the manner installed. The springs are too flat, more reminiscent of a TF truck with the axle mounted below the springs, where your original axle would have been mounted above the springs with a much higher arch and significant clearance to the frame allowing axle travel and spring load compression. Looks like they went for the extreme lowered look, but didn't account for actual driveability like Jerry says.
On your 1950 3100 the spring to axle mounting plates would have been mounted below the original axle/spring pack and the "pins" poking out towards the center of your truck as shown would have been the lower shock mount points and pointing to the outside of the truck. Those mount plates look like TF or later truck design with the pin in the center of the plate VS towards the leading edge on original plates.
As far as suggestions go, in a bolt on world I think you replace the springs with the proper arch height (including proper length spring shackles), remount the axle above the spring pack (will require removal and rewelding or replacing of spring pads on the axle tubes to the underside of the tube) and switching the spring pack to axle mount plate with shock mount to below the spring pack. This will allow for restoring the axle travel clearance you need and a way to figure out the readdition of shocks. From your pictures we can't tell if the upper shock mounts are still attached to the frame or not??? They would have been riveted to the frame on the outside face, forward of the axle center line.
One final note/suggestion. For safety's sake, it's recommended that you use new axle mount U-bolt hardware.
