Allan, I did mine on the frame but not bolted down... I think it might be easier if you had it off the truck... Be prepared to have help putting it back on the frame, it will be heavy...
I measured the bed dimensions... Making sure that the bed was squared when making my measurements... It was a bit difficult for my bed because it has not been together for 20 years... I did not know for sure where the cross stills went in relation to the bed and the frame...
I used two 4 x 8 3/4" sheets of treated plywood... Had them cut where I bought the plywood,( note that lumber stores that have their trim saw inside may not cut them )... The two pieces are the same size so the seam lined up in the middle of the bed, this made it easy to insert the plywood under the welded bed strips... I put a few coats of water seal/stain on them...
The cost for materials was about $ 150... I plan on using it to haul things so I went this route...
