I have the sing 2.5" in, dual 2.5" out cross flow flowmaster. The inlet necks up to greater than 2.5". I just took a tape measure to it and found that the circumferance is 9.5".
C=pi*d
C/pi=d
d turns out to be 3.02" so the muffler does have a 3" inlet if you cut the 2.5" part of the neck off.

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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....

To be installed eventually far far far into the future: Yours if the price is right and I don't have to ship :
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
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Flowmaster does make a 3"in/2.5"dual out 80 series, so they probably made that one first and put a reducer on it to 2.5"in.

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1989 Trans Am GTA, 5.7L TPI, TH700-R4, T-Tops, dual cats, 9-bolt rear, 3.27, Dynomax muffler, K&N
1992 Camaro RS, 5.0L TBI, T-5, T-Tops, L69 Dual Snorkel, K&N - Sold

 

The necks sticks out pretty far and looks integral. They might cut it off to make the other one.