Hy Skelflyer, I will begin my comments by admitting that the early 60s and V6 GMC engines are not my area of expertise. I believe from 1960 to 1962 the engines were mounted using a three point system, one mount center front of engine and one mount on each side of the engine at the bellhousing. The later 60s engines employed a four point mount system, as the big block Chevrolet engine was not released until 1965 I do not believe there are any factory parts to mount the big block Chevrolet engine in the early 60s chassis. As to engine weights I believe the GMC V6 and the big block Chevrolet V8 would be comparable in weight. I do not think the Chevrolet big block can be swapped into the early 60s chassis without some engine mount fabrication, hope that helps.