The only possibility I can think of is that the engine side portion of the mounts are different enough from the ones you originally had on the truck to make them not fit. A sbc is a sbc, and the holes are no different from one to the other (Unless you get into the LS stuff), so it has to be the mounts that came bolted to the block.

Do you have the entire old mount setup that was on your truck originally? If so, put that whole deal on and it has to fit. GM probably used a different engine portion to work with the later frame configuration.

Don