I was struggling over that gap at the tang. But it turned out the flat stock I was using was exactly the right thickness. So I cut a small piece, wiped it with flux, sandwiched it between the tang and the big piece, and just let the solder flow around it.
You can see the small piece sticking out. The big piece is way oversize, because I figured it would be easier to cut it off after it was soldered in, then to first cut to size, and then get it to stay in the exact spot as I soldered.
I did one side, then used the receiver lock pin hole on the other side as a pilot to drill the hole in the new piece. Then I soldered the other side and used the hole I’d just drilled as a pilot.
I did find there was a slight difference in angle between the recoil plate rear and the front of the NOS shoulder stock. I’m not sure if that’s because it was a type A, or if the stock was a little off. So I had to change it by about 1 degree to get rid of the gap.