here's pic of my 70 challenger. MAybe it'll better able you to understand.

The Pink dots would the drilled holes in the rail. Then need to Plug weld thru holes onto the face of the bolt that surrounds the threaded opening. Then grind down. This area is hidden once bkts are in place. Holes approx. 1/4" or larger, depends on how larger the bolt surface is that in the frame. Tricky part is retrieving the bolt & aligning it back into place. Sure beats cutting a plate out of opposite end (which is visible) & welding/finishing it nicely.