I have a 2004 GT and bought comp cams and it has a .550 valve lift. Will this fit my stock engine.

 

is that the comp 270 grind? if so yes.

 

When i called comp cams the Tech there advised me to stay away from any of the .550" lift cams to actually get the non pi cams. He said they've experienced alot of people calling and having PTV contact. He said some mustangs it will work on and some won't. it's 50/50 so stick with the .500" lift cams or non pi cams.
Me, i'm getting the non pi cams, just incase.
The 262AH, 270AH and 278AH are .550" lift

 

you can the 262 fine, the 270 work if timed correctly. But the 278's will not work in a stock short block. But there have been a good number of people with 270's that ahve PTV problems but almost all said they didnt truly time the motor. they just tossed them in.