Is the 80 series the same size as the stock muffler? Also are the stock tailpipes 2/1/2"?

1)Yes. 2)I just pulled one out the box and it has 2.5 inlet and outlets but I bet flowmaster has different sizes to fit your needs.

its a little smaller than stock. it needs some good shop to put it in. its not a simple job like a single in single out muffler.

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-Tas
'89 Formula WS-6

305, TBI, auto, 14x3 chrome flat based open element with K&N, Milodon 160* thermo, functional Formula hood, cross-flow Flowmaster, '99z28 rear pipes and tips....

Soon to be installed:
Hooker 1-5/8" 50 state legal headers, Dynomax 3" I pipe (PN 44063 and 43248), Catco 3" cat, and injector spacer.

Super GRK_Taz World
F-Body Dual Exaust
EFI & Intake Options
AOL IM: superGRtaz

i think it is smaller than stock. the catback comes in 2 main peices, a muffler, and 2 tips if i remember correctly. i did mine myself, used exaust clamps and permatex to seal all the places that connected it and i gutted my cat while i was in there. make sure you keep the rubber thing on the muffler that holds it up or you will get loud screeching. and, uh, dont um weld the tips on till you drive it around for about 3 days, the whole system settles in kinda in a few days and everything moves. only weld about 3-4 days after you install the exaust. THEN you can worry about how the tips look. oh and after it settles in ALSO remember to Re-tighten all the clamps if you used any. remember you may have to remove you exaust later if you do any TRANNY work, so its a good idea to use clamps and a sealant like permatex.
Look here:
http://www.geocities.com/kingtaling/main.html

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1989 Camaro RS:
Shift Kit, Flowmaster AT, Gutted Cat, Man. Lockup, Foglights ;-)
15.4 @ 97 MPH
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1983 Iroc Z-28 Camaro:
355-CID (280 Duration; 225 @ .050; .466 Lift) 2.02/1.60 Valves In Aluminum Chevy Bowties(fully ported); 1.52:1 Ratio Roller Rocker arms; Forged Flat-tops w/ valve rel. Pistons;Performer RPM; 600 Double Pumper Holly w/ Jet extensions, Notched Floats, Titanium Needle & seat; Mallory Ignition; High torque mini starter; Carter 7 PSI race pump; MSD Billet Distributor; One peice stainless valves; 3.23 Borg Warner Posi; 4-speed Muncie M-22; etc....
I can go on for days.
12.4 @ 138 MPH