I remembered we have this schematic drawing of the 48-52 Express bed. It gives you an idea of how the rear metal cross member fits into the bed. Stu

 

Thanks again Stu I found that diagram too but its a bit difficult to make out you say it bolts on top of the frame rails? at this point im not too concerned with originality I have to get the bed onto the truck so it doesnt look so ripped apart

 

This is the first time I've really studied that part of a bed. Still a long way away for me. I wish I could make a line diagram of the cross section similar to the above middle wood cross member, but I'm working off the iPhone these days and don't know that I can do that.

Anyway, the goal is to have the piece span the full 54" width, weld into the bed sides, and support the ends of the bed boards. Again, the old scars should help you see what was there. Here's one of my trucks that still has its complete original bed.



These next two pics are of the bed from my project truck. Standing on end in the shop. You can see that it's basically a U channel, with a flat piece on top completing the box, and an angle iron piece on the inner face supporting the bed boards. The U channel is 3.5" tall and 2.25" wide. The flat top is 3.25" wide and aligns with the inner edge of the U channel. That leave a 1" outer lip. The bed boards are supported by a 1" angle iron aligned to allow the bed boards to meet flush with the upper flat plate.




The rear bed is then anchored to the frame with a long bolt through the top lip of each of the frame rails. The original bolts are special square drive flat heads, but I doubt you care about that. Here's the pic of the bed attached on my parts truck. Stu

 

Stu, here is the Mid-Fifty rear cross member, you can enlarge the pictures there to see if it's the same.
1950-72 Ford F-100 Long Bed Crossmember, Rear Sill, Original Style
They also show wood mounting block to go inside the crossmembers? http://midfifty.com/item.php?INV_ID=9985
Here is the section from the catalog with all the crossmembers

 

That rear U channel looks close. Might be a good starting point. Not sure what the wood blocks are for, I guess inserts for the U channel. Edit - I just repeated what you said. Duh. Stu

 

Yea, it says they "go inside crossmembers holding them above frame."

 

Hope we hear of somebody trying it. I don't think it would be hard for the o.p. to fab his own, as he's said is the plan, but new pieces would make for a clean installation. Stu

 

Thanks again Stu for the pics your help is really really appreciated. Thans harley looks complex enough that it might be better to buy it but the hard part isn't really the $250 its the extra $100 or so for shipping/doody etc. So the one part is basically just a valance I could figure that out later. whats annoying is my rear rails have been hacked off and the crossmembers are bent from someone using them to pull with. I think what I would do is start with a peice of tubing the correct height what is the measurement from top to bottom where the bolts for the frame run through? and how far down is the angle iron "lip"? I also need to know how wide the framerails (outside to outside) are so I can rebuild the rails, instead of pulling the crossmembers for now I'll just build a C shaped peice from channel the rear most piece being like an extra crossmember then tack it to the frame til I can get the cossmembers straightened etc........and why are you calling me o.p.? I never called you gomer or barney or andy...lol

 

You don't have your location posted so I have no idea what part of Canada you are from, but as I posted earlier, these may actually be made by Pro's pick in Ontario. Perhaps you could call them https://www.pros-pick.com/contact.html

 

If I'm following the first question correctly, the U channel and boxed upper plate stand about 3.5" above the truck's frame rail. The flat support lip of the angle iron should align with the lips of the bed sides and should allow a full 1" thickness of the bed boards. The rear frame is 34" wide. Stu

 

Thanks again Stu you're really helping rescue this old truck! I should have enough to go on now with all the pics and data you've provided I appreciate it..... I'll post some pics as I put it together