Do you have the iron GT40's or the aluminum turbo swrils? Generally speaking you can run more timing with aluminum heads than you can with iron heads - don't ask me how. I think PKRWUD answered one time, but I can't remember.
Anyways, if they are iron, try 13 or 14 degrees with 92 octane and see how that does for you. If aluminum, try 16 or so. I used to run 17 with my TFS heads and 94 octane with no pinging - ever.
Gains are going to be minimal, but definitely worth it. Maybe 3,4 or even 5 hp, every car seems to react a little differently.
Hope this helps.
E
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1991 5.0 LX Coupe - 40,750 miles
331 cu. in. / Tremec 3550 / BFG Drag Radials
12.22 @ 114.31 mph - w/1.89 60'
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