If you put a 3/8" bolt in a 1/2" hole, you have slop. Allow enough slop, and even new mounts can have different clearance than the old mounts. There is also the possibility that the thickness of the new mounts is not as much as it should be, so they allow the whole things to sit lower in the frame than designed.

IIRC, pre '70 big blocks have a steel mount that bolts to the block, an isolator that bolts to the steel mount which then bolts to the pad on the k frame. You have variation in the mounts. You have variation in the location they bolt into the mount (not much, but some) and variation where they bolt into the pad (I believe this may be a slot, not a round hole, but its been a while since I've worked on one). That's four possible variation points, per side, for a simple bolt in engine mount.

Relay rod I assume is the center link. If the center link is hitting a corner of the pan with the engine lifted, your engine is not level or your linkage is not moving in a flat plane. There may be a difference in the assembly of the mounts from one side to the other. The engine may not be set into the mounting slots equally.

Alignment of the steering linkage itself could also be a factor. I have found numerous Mopars over the years that had shims behind the steering box where it bolts to the K. You can alter the plane the linkage operates in by shimming the box to move this plane up or down. There also is the possibility of idler arm and its bushings not duplicating factory position, or the bushing is worn/bad and allow unwanted motion, or its specific mounting location is just slightly off. Or, in the case of ultimate tolerance stack up, a little bit of everything above is off, just a hair, but add them all together and now something hits.

Personally, I'd much rather install a 1/4" shim into an motor mount or try moving some other pieces of the puzzle around to create the necessary clearance before I'd hammer on a $400 oil pan.