Bill,
You are on the right track with the slot AND the raised piece as described. Of the originals I've seen and I believe the Kirkham and Continuation shelbys they use a slot for the swing arm to slot into, then a piece of steel rod - I believe 3/16" steel bent like your piece shown. The loop is designed to be above the floor enough to catch the swing arm and then the swing arm cams it down tight. Shape the rod with a raised center as your drawing above shows, 90 degree legs, and bent 90 degrees on the bottom with loops at the end so you can bolt it to the floor, front and back of the slot. If you get it close, you simply shim it to fit. If it isn't close, you make make a new piece and try again.
Of course, you could go the elaborate route and either drop the tank, take out the trunk floor piece so you can stick your head in there and measure everything, then put it all back together. The clay idea was best, though, IMHO.
Don't let this stuff get to you or you might as well dump it and take up reading novels for a hobby. I know, I had to make my door hinges 5 times and they still aren't 100% right.... also built my floor pedal box, hood and trunk hinges, control arms, shock mounts, trailing arms, and a bunch of other stuff and none of it was right the first time, or the second....
Bob