If you have a 99 mustang GT, then you have a PI headed 4.6 rated at 260HP.
I would say you are running NO faster than a high 15 at best. My reason is because your truck weights as much or more than my ex and we have similar mods, but I have a 5 speed auto as opposed to your 4 speed and I have 3.73's as well. Like I said, my cousin had a 4.6 F150 that was slower than dog piss and my ex would eat it alive. Take it to the track and let us know what it runs Also, have you raced the Lexus yet?
Edit: I looked at the 03' you mentioned- He had a 75 shot of nitrous, CAI, and catback exhaust. He said he's running 93 octane so I assume it's tuned for the nitrous.
i thought that the PI heads didn't come out till 2001 or atleast that is what i have been told by most guys. and no i haven't ran it yet. its been raining all week here in orlando and my friend drives his escape on crappy days. just as i drive my f150 on crappy days. i have ran the escape and beat it tho and it has a V6 and its a 4x4 nothing special about that tho