Need help identifying stepside bed part

[Reed]

I found a mostly good condition stepside bed for my 76 D100 shortbed truck but the original wood is long gone and it only has three of the five bed floor support sills. The front and rearmost sills are present, but I only have one of the three cross sills that go in the middle of the bed. This is my first truck so I can't tell by looking at the sill where it goes on the bed.

Here is a picture of the profile of the mount for the sill:

Yes, the bed floor was replaced by pressure treated lumber bolted to the cross sills. I didn't do it.

Can anyone tell by looking at that picture what position that cross sill sits at under the bed? If the five sills were numbered 1 to 5 with one being by the cab and five being by the tailgate, what number would this be? I want to replace the fleetside bed with this stepside and I need to know what other support sills I have to track down for the installation.

Thanks for any help you can give.

 

[7mopar]

If the front and rear sills are attached to the bed set it on the frame and start measuring. If not attach the front and rear sills to the frame sting line the top and move it to were it fits. As you know the top of the frame rails are not straight and without exact measurements and marked rails there's a great risk if them not being located right.

 

[Reed]

OK. After studying the profiles of the replacement supports closely on www.dodgeconnection.com I believe this is the third brace back form the cab, or the middle-most brace for the bed floor. I will have to measure to be sure once I get the bed on.