Welded the retaining nuts to these plates and bolted them into place on both sides.
I gave myself some extra room to move the mounting points around with these 4x4 plates.

The plan was to set those points where I made the "critical cuts" on the Caddy right on top of these plates, but I must have measured wrong because it is just way too narrow after measuring again.
Looks like I will cut-off these legs that I left in front of the Caddy axle tunnels and weld in some horizontal plates.
Then I can span that distance between the plates with steel tube.