I can't say for sure on the poly, but I've installed a 318 Magnum in the G/Friends 55 Desoto that had a 291 Hemi (gone) using the early Hemis motor mounts & brackets.

I had to modify the R/S mount bracket and add some metal to have a place to put one of the bolts. (the front lower one).
The L/S was a bolt on.

I had done a mock up with a late 60's 318 and the mounts bolted right on, but the Magnum engines have one of the 4 bosses machined off the block. (why I had to add metal to use the other boss).

R/S pic.


L/S pic.

There's a good chance it'll be a similar thing for you.