I know this thread is old, but thought I'd reply.... Joel Haffner was my dad. (he passed away in 2015.) Others are correct, he used the convertible top from a Cutlass. Jim Novelli did the conversion, and his work was perfection. I still have a big stack of pictures somewhere of the conversion step by step. There was a detailed article about it in Pontiac Enthusiast - Volume 4 Number 5 - September/October 1998. He had the article framed and displayed it at car shows. (pic attached)
He put a 455 in it along with with a bunch of other things that weren't stock for this GP. Although the upholstery was in decent shape, he replaced it with white leather with the same pinhole pattern of the original vinyl. I think he also installed a dual shifter, although that may have been another car I'm thinking of. The car was absolutely gorgeous and I wanted to keep it but money was needed for his care. I unexpectedly cried a little when the guy picked it up and drove off - it was the last of the many great cars he had over the years.
