So the only thing that goes between the bed crossmbers and bed are those 8 pads? It's wood on crossmembers.. And in the last pic, it looks like wood on frame

 

On my 49 I made my own pads from an old piece of conveyor belt I had laying around. Basically some chunks of 3/8" (well maybe 1/2" thick) rubber with a hole punched in it.

I don't know what was there originally. I know on the newer trucks they used wooden blocks. Here's a drawing of the 57-60 blocks;

 

From what I remember on my 50, I removed the steel floor and 2x6" wood sub floor .Then I put the rubbers between the frame and xross members, where the bolts can run thru them, then used 1/2"x 2" wood around the edge of the bed and across the tops of the xmembers as a spacer to bring my new red oak floor up to proper level. (and hide the old tack weld) I hope this may help , as I cant seem to transfer the pics I have. PS I believe I got the I/2" x 2" spacing wood at Classic Haulers. "ALSO" I have a couple pics in my album #2 if you care to look.