thres so much inaccuarcy in this statementyou need an adapter kit to put the pi intake mani onto non pi heads, thats why most do the heads at the same time.
imo the heads are the main thing to the swap.
npi heads are the weak point in the 98- motors. with the pi heads your upping your compression as well as opening alot more breathing room in her. you can almost pull a 25hp gain from the pi swap alone.
a pi swapped 98 gt will put out more than a 99-04 pi motor hands down.
you wont see the same gains from just swapping the cam as apose to the heads,plus for the price/work of swapping your cam its almost smarter to do the whole heads.
either way hope the swap goes well!