Eliminate rear bumper rubber filler on a 74 Duster

Does anyone know what the head size of the large bolt on the front side of the back bumper shock is on 74 Duster and can you tell me if the nut is welded in place inside the frame rail?

I am eliminating them and it's larger than 7/8" ( I had a small set of sae 1/2" drive sockets which have vanished)

Thanks for your help!