Steve,
Don't pay up yet. I think you are correct. But, it was GM Chevrolet division (not GMC) that continued production (or, maybe just sale) of the 235 (and maybe the 261) into 1963. I think it was used on the standard production NAPCO-style 1963 4-wheel drive trucks. I'll see if someone with documented information is willing to post information here.
Tim
