Never paid much attention to it, but after pulling the busted 428 from the GTO, I noticed the exhaust pipes looked only about 1.5" - like what was on a 69 350 LeMans. Was this stock back then, or do you think the previous owner stuck some salvage yard parts on it? I would think this HAD to reduce power with the original 400.
Read some on here and saw Doug's headers go right on (I have 67 GP 400 with Hedmans and hated seeing where guy hammered driver's side to get 1/16" clearance from steering gear). Then read reviews on Summit - one says the oil filter can't be changed without pulling the OF housing, and all but one other say there were manufacturing fit issues (which Summit promply corrected). Is this true? The Doug's look solid with heavy flange.
Alternative (if I keep car) is using a 2.5" Pype's Performance downpipe to keep stock appearance.
Not sure but I believe most headpipes were at least 2".
I have a 68 Lemans with Hooker Super Comps D port headers. Believe me when I say this, they are by FAR the best fitting Pontiac A body( GP same but longer frame right?) headers but they are big and may not be the right choice for your build. Note I did not say "fun to install" because any header in an A with a Poncho engine is not fun. Make SURE engine mounts are new as well. You need the clearance.
Also, get an IMI mini-starter, you'll need that.
You'll need to removed the oil filter mount and any clutch linkage or trans linkage. The engine will need to go up and down a few times to get them in. Use the smallest headed bolts you can for the flanges and get a grindable short 7/16' wrench as one bolt on the driver's side is a little tough.
CAREFUL OF THE DISTRIBUTOR!
If you've got enough engine there's nothing better than Super Comps. Oil filter will be a 1258 NAPA/WIX size and is easy to change when everything is back together.
In the end, NO HEADERS GO RIGHT ON IN THESE CARS. Only manifolds do. LOL