Maybe this will help. Small block to Big block conversion mounts.
Hard to tell from your limited pics, but they do appear to match these.
Conversion Motor Mounts Small Block 318 to Big Block 383 440 Mopar A-Body Dodge | eBay
May have missed it, but aside from the cast rear main seal cap shift issue, is your install past/planned also including the seal position rotation offsetting in relation to the seal retainer/block flange mounting area?
Found another very interesting read over at moparts.
440 Rear Main Seal Leak--Almost Suicidal!! UPDATE
Excellent points and primary solution appears to have been provided on the first page by poster, Monte_Smith...
Not sure but think this may be what you are referencing:
Mancini Racing Billet Rear Main Seal Retainer
"Fits: Big Block, Mopar
Billet aluminum rear main retainer. It uses double o-ring on each end to seal the cap to the block. This retainer is designed to hold the proper diameter to clamp the...
The guys over at Engine Swaps Simplified did a great job of putting together a K-Member ID page, check it out.
Should be what you need to positively ID your unit.
K-Member ID Chart