I ama LS1 guy, but trying to see how hard it is to get 400 to the wheels out of a 2003 GT. please advise, with out blowing the car, want all motor, everyone tells me this is not going to happen and be a street car???

not going to happen. You'd need a high compression pistons, 4v heads, built short block, and ridiculous cams, and even then you'd fall short of the 400 whp mark.

shm short block high compression
4v b series heads
stupid cams
stroker crank
jesus i cant remember mustangs that well haha..
but i assume u mean a 2v

shm short block stroker
patriot heads
shm cams
reichard racing intake

some other tasty shit

sean hyland has a book onthis type of stuff. it got me through high school haha, its a worthy investment.

Not going to happen with the 2V motor the car came with. Possible with a 4V out of a 99/01 Cobra or better yet, a 03/04 Mach 1 (better heads).