I don't beleive that there's any differenc from short to long in the engine mounting, but there might be a difference from Chevy to GMC. That GMC was originally equipped with a really BIG V-6. It's mounting points were most likely different from the I-6 or V-8 that would be in a Chevy. I seem to recall that the last GMC I looked at, the engine was mounted somewhat forward from what a Chevy smallblock would be.
You might nose around Summits catalog a bit, I think that they have those mounts you are looking for.


Bill Burmeister