On your first page you had a picture of this reinforcement bracket you found on the later front end. I found the same reinforcement on the '76 C30 SRW that I took the disc brake parts from for my '67. I believe they added the reinforcement, because the brackets on the outside of the frame that the upper A-arm bolts to is a lighter weight pressed steel piece, rather than the former Forged Steel bracket, riveted to the cross member.
I used my '67 cross member with the Forged brackets and added the inside reinforcement brackets as well. I used the six cylinder coil springs, but everything else including the brake lines and forward mounted Combination Valve is from the '76.
I realize that you are well past the cross member, but the original, later Combination Valve mounting on the small cross member, under the radiator might be worth considering. It is hardly noticeable down there and there is already one usable mounting hole. Later you will need easy access to it, to hold the Metering pin in for bleeding.
Just a thought.