Bob,
Depending on how you have your fuel tank installed, you might be able to run a bar from roll-bar mount on each side and mount the harness to that.
When I had the body off the frame I looked at it and came up with a post I welded to the frame and a gusset on the back-side. (behind the post) They are on either side of the fuel tank "area" and wide enough to allow wiggling the fuel tank in the body. I had threaded inserts welded into the post(tubes) so I could incorporate a (possible) tank mount and mount point for the seat harness behind the "Firewall" fiberglass. I had to punch holes in the fiberglass as I mounted the body on the frame and lowered the body on top of the 2 vertical posts and gussets.