Let's set some things straight here peeps.
1) NO WBODY car has duals...we have a single downpipe to a "Y" that goes into 2 mufflers. A Silverado has duals...we have a Y Pipe.
2) Anything with a Series II or Series III 3800...L36, L67, L26, L32...has the same catback (as long as it is the dual outlet one)...all you have to do is hang it.
3) What does factory freaks have anything to do with a dual outlet catback? Nothing. Factory freaks are the definition of fast, bone stock cars...not what options they had from the factory.
4) Unless your meaning Special Edition GT/GTP's...no SE came from the factory with dual outlet exhaust...hence the different rear bumper on EVERY SE.