While dry fitting new bed angle and strips to the bed structure and sills it appears to me that the second sill that goes over the fuel tank is bent. See attached photo. This is the original sill. I think that its present shape would / could prevent the wood boards to lay flat. If its bent I think I can straighten it out.
The cross sills should be flat. That one is definitely bent and will not allow your boards and strips to set evenly.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
Definitely bent, like Dan said. I see it has the cutout portion for the fuel tank to fit under the bed. Should be able to carefully straighten it. You may need to apply some heat to get it done properly. Heat will shrink the stretched bottom flanges. Replacement cross sills aren't that expensive if you have trouble with it. Might be a little harder to find with Mar-K out of business now. Wow! I just checked and that particular cross sill with the tank cutout is almost $80 now. I'd be straightening it too.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
Ok gents I must confess that I did not do as you recommended. I came up with a alternate solution try to stretch the sill using weight. Not having weights to attempt my idea I used the trucks weight to see if the sill would stretch. So I jacked up the truck by the differential and placed the bent sill under the tire (see photos.) It did stretch but would curl up again when the weight was removed (as expected). So I decided to cut the sill at approximately the area where I considered the bending originally occurred. Once I did that, I placed the sill under the trucks tire and this time the sill stretched to its flat form.
See attached photos.
FOR YOUR INFORMATION : I will be welding the cut areas once I finish dry fitting the bed.
That's one way to do it. You may find that it's slightly shorter than original, but probably not enough to worry about. I'm seeing a slight kink on the top (yellow arrow) which would shorten the overall length, and a stretch below that about where you made your cuts. When bending steel, there will always be some springback, so you need to overbend just a bit to get the final results you want. But that'll work.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
Kevin, true forcing the steel to change shape will have some springback but by me cutting slits into the front and aft vertical sides and not cutting the top flat horizontal surfaces does not change the overall length of the sill. Making the vertical cuts allows the sill to reconstitute itself to its basic nominal length and then filling the vertical cuts by MiG welding lets the sill retain its flat and length.
