Derek, I apologize, my home computer is shot, so I havent' had a chance to put my two cents worth in on this. As Dallas and Robert have already said, the FT-317 was originally designed for power supplies in the Medium Tank M4, etc. It is a wall mounted unit, see photo below (I hope).
I have a set of 11 pages of "Restricted"
"Instructions for Installation of Radio Set AN/VRC-3 in Medium Tank M4 Series" (22 March, 1945). This information came from Ron Germain of St. Clair, Michigan - who is (at least to me) the guru of BC-1000 radios. Ron sent me all this detailed information with the intention of reproducing the FT-317 mounts for the BC-1000 radios. Robert Downs, of Houston, was kind enough to loan me an original FT-317 to copy. I have made detailed autocad drawings of the FT-317, and even found an exact duplicate of the rubber shock mount. I --- COULD --- build FT-317 mounts, but I don't see the need.
The FT-317, as has been said, is a wall mounted unit, which attaches with a swinging swivel arm at the top, and an inverted angle at the bottom. The overall height of the FT-317 is far too tall to fit inside any vertical section of a jeep, and the inverted angle at the bottom does not lend itself to mounting on a floor.
As to the modifications of the FT-317, according to the "Instructions" it primarily consists of a wooden block (4" x 10" x 1") shown as a 2Z8807-76 in the "Instructions". I am lead to believe that the flat stock bracket around the FT-317 is changed out for a different (deeper) bracket (possibly as per drawing SC-D-7327). The FT-317 also required for any application a bracket as per drawing SC-D-7393 which provides the pivot point for the top of the FT-317 radio mount.
All of this, at least to my way of thinking, made the FT-317 virtually unusable in a jeep or smaller vehicle. It could possibly have been used on a WC size vehicle. Hence my "redesign" of the FT-482-A.
All of this finally gets around to answering your original question. The modifications to the FT-317 consist of a wooden block and a different flat steel strap. The other unasked question would be the modifications to the FT-250, for which I also have a drawing, thanks to Ron Germain. The FT-250 was physically cut in half and reduced to 12-1/8" wide from it's design width of 15-1/2". Then they welded a bolt bracket to the FT-250 and designed a flat strap to go over the top and hold the BC-1000 into the FT-250. It also had a pair of "legs" welded to the bottom, so the whole thing could then be turned upside down and mounted with the FT-250 "bottom side up".
Derek, I will copy the "Instructions" and post a copy to you.
Tom Campbell - Birmingham, AL