You cant take the mounts off of the old motor and put on the new? The holes in the block should be the same even if the mounts are different.
But maybe he sold you a 232 instead of a 258 (4.2L)? That would cause the change, if thats the case. The 232 straight six was a common motor in the mid 70's, much more than the 258. They look close to the same until you tear it apart.