EST..1/4 mile time after PI headswap and 4.30's install

depends on how well you can drive, you should trap about 103-104mph though.

now with a good set of tires, decent driver and off-road pipe it should go 12's

I take it you're using the stock PI cams found in the 99-04 GT's. I'm also taking it the cylinder heads are not cnc ported and polished and the valves are stock as well. If that is the case, then the only extra power you're going to see over the 99+GT's is going to come from the dumped flowmasters, which isn't much. If you get a tune with the PI head and cam swap, then the tune should give a bit extra power as well. The 4.30's won't add any power, but will help in acceleration. IMHO, don't be surprised if all you run the first time at the track with these new pieces is high 13's-low 14's. Not saying you don't have the potential to be faster, but you have to get used to the new gears before you'll start knocking time off.

EDIT: I just noticed you haven't mentioned anything about drag radials or slicks. You'll need a set to grip traction with the new power you're going to gain and the 4.30's. If you don't run a sticky tire, then you're not going to grip worth a crap.

I believe you'll see pretty good mid-high 13s, but you'll definitely need some good hookup with some DRs/slicks before you start getting anywhere near or below 13.00.