The only thing I can think, is that the 60 is vacuum controlled rather than electronically controlled. The swap would be too much time and effort.
That Bonneville, though... I've always wanted one myself, and they are far fancier than a GP. Lot's of people on here have had both, and most prefer the Bonne. Cat issues seem simple to band-aid, Amazon/Ebay a new one for 50-75 bucks and throw it in.
It might not solve the original problem, but it will buy you time to get it figured out.
And $900 bucks for a running car isn't bad, especially if it's clean.
