The opening of the ball flange on the collector is a pitiful 1.9" going into that sweet 2.5" downpipe. Hooker's excuse for this is to "prevent leaks".

My solution:
With headers installed, did a fit check mounting y-pipe and marking orientation. Then, took 3.5" high-speed electric angle grinder and ground perpendicular to mate marks on ball (starter out - no room on passenger side otherwise). Opened them up this way to ~2-1/4" (about the ID of the downpipe), still plenty of material left for the seal.

Tried die grinder first, didn't touch them (forming that ball must really work-harden that stuff). Angle grinder did the job, though.

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4 (restalled TC). Ported World 305 heads, Crane PowerMax cam. ZZ4 intake, oil pump, pan & baffle. Accel HEI SuperCoil & module. Hooker 2055 headers, 3" Catco cat & 3" catback w/dual-opposite Flowmaster. 2.93 limited slip, Spohn SFCs. AMSOIL syn lubes bumper-to-bumper. Daily year-round driver. Best ET, speed TBD...
57 Bel Air, my 1st car. '66 396, 9.7 CR forged TRWs, Weiand Action+, Holley 750VS w/4150 conversion, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 700 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).

 

Since a lot of Mustang headers have this type of flange I asked at www.corral.net . They said take the headers off, see where the carbon buildup is. That will show how much is in the way of the flow and grind accordingly.

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-Tas
'89 Formula WS-6

305, TBI, 700R4, P.A.W. 14x3 open element with K&N, Milodon 160* thermo, functional Formula hood, cross-flow Flowmaster, '99z28 rear pipes and tips, Hooker 1-5/8" 50 state legal headers, Dynomax 3" I pipe (PN 44063 and 43248)
Super GRK_Taz World
F-Body Dual Exaust
EFI & Intake Options

 

Being lazy, I only wanted to jack the car up once. And, getting the cat connection loose after being clamped/welded would make it all more difficult.

However, if the y-pipe is ever dropped for whatever reason, that's a good idea as well.