kenne bell stopped making 96-98 kits a while ago, yes you can stick a 99-04 kit on a 96-98 but that's only if you have P.I heads. As for positive displacement yes they are nice because you get lost of boost early, but 1200rpm is to low. I test drove an 03 cobra and it didn't start till 2k and in my old saleen charger it started at 2k.
I'm not trying to talk him out of a kenne bell i'm just saying that money can be better spent elsewhere. If he has 7k the most he could do is get a kenne bell 5.5k then maybe a set of cams. If he gets a used vortech for 2k or even new for $2800 then he'll have $4200 left over (if bought new) and 5k left over if used. He could get his whole engine forged, get his heads ported with cams and all installed for $4500-$5000 and then take that S trim up to 500hp and whoop on any stock engine mustang with a kenne bell unit for the same amount of cash. He also mentioned track car so centri is best for that, a kenne bell will only cause bad 60ft times unless you have full suspension, slicks and so on. We also dont know whether his car is a stick or auto that makes a difference too.
I'm not saying go out and buy a centri, im just saying that sometimes for the same amount of cash you can get more for your money. If you really want a kenne bell then i would highly suggest buying one, they are really great blowers. Just remember that that stock bottom end wont last long pushing 400+hp/400+tq on those weak rods especially with full boost by 2krpm. That kind of power is meant for at least 3k rpms. If he gets a forged engine not only is his miles 0 but he has an engine that is strong. If anything he could forged his engine, buy a vortech, run it on his car at 500hp till he gets the money for a 2.6 kenne bell then sells his vortech for $2k and add that to the money he has saved. Either way you wont be disappointed, just remember it'll cost more to fix a blown engine over a working engine.