Hi DLDGTO,
Yes you have "D" port heads, the outer exhaust ports are oval and the inner 2 are "D"'s back to back. There are a bunch of companies that make headers to fit your 66 GTO. Most of the "D" port headers are universal in that they will fit most of the "A" body cars (like you have 64-72) and will also fit second gen firebirds (70-7?) not sure on the end year, but you will see when you look them up. "D" port headers have quite a price range from just around $100.00 - to around $500.00 depending on brand. There are also 3 tube and 4 tube headers for the "D" shaped heads, but stick with the 4 tube, much better performance.
Pontiacs also have what is called Round port headers where all the exhaust ports are round and take a totally different header.
Check out our sponser "Performance Years" they carry a big line of headers for our cars.
Hope this helps
Mark
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1965 GTO: 467 (Built By Dan Willever), 400 Block, Eagle 4.25 Stroke, Eagle 6.8 Rods, BRC Pistons, Custom Grind Hydraulic Roller Cam, Edelbrock 72cc Heads Ported, HS Rockers, Doug's Headers, Edelbrock Victor 4150, QTF 850 Carb, AutoGear M22 4 Speed, SPEC Clutch and Pressure Plate, 12 Bolt 3:73 Posi.
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