66 GTO Exhaust
 

I am open to recommendations on improving my exhaust system on my 66 GTO.
My down pipe from exhaust manifold is 2". Stock 389 4 barrel.

2 1/2 Mandrel bent head pipes are your biggest bang for your buck. I prefer the ones from Ram Air Restorations myself. I'm sure others will chime in with their preferences. Dynomax mufflers are a favorite here, and 2 1/2 crush bend or mandrel tail pipes are both fine. Your head pipes being mandrel bent is what helps the most in your situation. I don't know if you are wanting to go this far, but i really like the Ram Air exhaust manifolds from Ram Air Restoration with the 2 1/2 outlets too.