The Stovebolt Site Forums The Shop Area Making a Stovebolt Bed I need help with bed.

I don't know the depth of the original cross sills, but I believe they're quite a bit taller than the unistrut that you have on yours. I'd recommend that you ditch the unistrut and buy original cross sills. Mar-K is a good place to get the stuff you'll need.
The original shock mounts on '50 and later trucks do extend slightly above the top of the frame, but not higher than the hump over the rear axle.
Here's a pic from my '51 before disassembly:

[on edit] Taking another look at your picture, it looks like the unistrut is mounted on top of the angle strips on the bed sides. The cross sills go underneath the angle strips. Your bed is currently too low, so your fenders also won't fit correctly (the bottoms will be below the running boards.) Also, it looks like your shocks are mounted nearly vertical. For best performance, they should be at a 45 degree angle off vertical. Or at least 30 degrees if 45 doesn't work well.