OMG,
For your stock 389, I don't think you could tell the difference between 2.5" and 2.25". However, plan for the future as reading the Forums will soon have you lusting for "more power". 
My signature 66 GTO has Doug's headers, 2.5" X pipe, 2.5" 3 chamber Flows, and 2.5 full length tailpipes. My wife and I drive this car cross country to POCI and GTOAA national events and I dragrace it monthly in the Pontiac Drag Days Series. You need to decide how important "correctness" is and what your objectives for the car are.
Les
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Les Iden
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'65 Buick Sport Wagon Custom, 340, T350, 3:23
'66 GTO Post/468, 700R4, 3.31 (Mike's as of 9-16)
'68 Grand Prix/455, dual AFBs, T400, 2:93 posi (sold)
'72 TA tribute/461, T400, 3.08, (Russ's as of 9-16)
'97 Mitsubishi Eclipse Spyder Turbo, Konis, 5 speed
'09 Torrent GXP, nav, Sun & Sound pkg., Bilsteins
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