Hello
@KevinGaus ! On page 2 of this thread, in
this post, I found my (I think they were 1/2") u-bolts from TNJ Murray (P/N: 11609677) but the link is now dead.
Searching TNJ Murry for "u-bolt" it appears they have u-bolts for the LWS gun ring system, but I'm not sure of the outside diameter of those "legs" so I'm not sure if they'd fit an M66 or the holes in the back of your cab:
https://tnjmurray.com/12450097u-bolt-weapons-kit-m923.html?fireglass_rsn=true
My M66 gun ring legs are 2.25" in diameter, and the whole setup weighs somewhere around 600 lbs. Since you'd be using at minimum 4 u-bolts, I would think you could use virtually any 1/2" U-bolt of sufficient length with a ~2-1/2" inside radius. The back of the cab isn't very thick anyway.
You could also contact:
https://www.bmgparts.com/
if you want to find more hard to find parts, or more "historically accurate". When I talked to the fellow there, according to him, the older M-49 gun ring's u-bolts were a flat strap on the back, curved portion to better grip the gun-ring leg. Obviously premium/desirable parts there.