Just spoke to him and he said he used a set of mounts (body and engine) from the mid-80's EFI 460 trucks, and adapted them to the body of his '56. That was only a stock LPG motor with a tunnel ram though, not a 557... so maybe the body mounts with the hardcore Bronco mounts above might get you home?

I think your bay should be the same as mine, and he reckoned it would be even easier to do my '74 than doing the swap in his was.