I have found that the M-series trucks came with 1" metal spacers and the 1" rubber body mount bisquets, The wide track and later FCs used these rubber mounts without the spacer and the Narrow tracks used a thin pad. Most Jeep shops sell the stock replacement rubber mounts, but after a year they start to crack.
Because the design of those original rubber mounts was for use as one of eight mounts to be equally spaced under the wieght of a CJ or an Overland Wagon or truck. It is not the best body mount to support the wieght of a very heavy FC cab with only 4 mounting locations. The stock design of an FC is to use the pad between the frame and body only, in front mounting, and the addition of double springs and washers on the rear.
I suggest that you discard the all threaded rod and use real grade 8 bolts and nuts instead.
If you want that look of the extra inch, then I suggest using 1" steel aftermarket body spacers.
Doubling the rubber pads makes for even more of a potential problem because it allows more side with the already small 3/8" bolts allowing movement.
So here is my solution, a non original non conventional way that I set up the front, I use hockey pucks or Semi trailer rear bumper rubber donuts on FCs. I drill the pucks or bumper pads out to have a 7/16" center hole, then I use thick steel construction washers that have 7/16" holes, and you probably already guessed, I use only grade eight 7/16" bolts. I use a dab of wheel bearing grease on the bolts and body hole.
The cab body of an FC is very heavy, but adding the safety of the people is worth more that the cost of good bolts. I had to reread that you had 5/16" all thread for bolts.