MB, I don't know much about the FE series motors like the 390cid, except it may be harder to find parts/upgrades for it than a 302/351W.
I put the c-6 tranny in, and now have an NP208 t-case bolted in the Bronco. The F-350 single tranny mount was too tall, and the C-6 hit the floorpan. I re-used the original Bronco C-6 tranny crossmember and mounts. Ford wanted $110 for new bushings for the x-member, so I bought some poly's for a 78/79 Bronco from Bronco Graveyard for $35. These bushings are the same height as the OEM units, but have a smaller-diameter center. I bought some big flat washers that fit the poly's, and welded those washers to the underside of the x-member. Tranny now sits at stock height, with two-fingers clearance under the floor. I can't post pics here, but have a couple at superford.org under "460 swap". JSM84