Thanks for the info & I think its the first cross sill support that's back from the front end of the bed. I had the 2 original rear supports and bought the front 2 from LCM which are for the 72 & up stepside beds. One matches the frame holes & the front one seems to be too wide, but we'll make it work.
We've almost got the complete frame off resto of the NV big block D100 I got & have changed the behind the seat fuel tank to an in frame fuel cell & the battery also to inside the frame. It was originally a short bed Sweptline & the guy I got it from changed the bed to a Uteline, so that's the issue. We also made a custom hard wood tongue & groove bed without the metal bed strips, so that seems to match all the holes in the cross sills. Once we get the front sill mounts moved in an inch, we should be fine. Seems the 60's Dodge trucks are rare as hen's teeth & its hard to find correct aftermarket parts