The 99 heads would give you about the same performance increase as ported stock heads.
If you go the 99 head route, there will also be other small modifications that will need to be made to your engine to make them work properly (I do not know the specifics...)
To be honest, whether you purchase 99 heads or port your old ones, you're probably looking at comparable costs for comparable gains.
A dry 75-shot will be about as safe as you can get with forced induction. (There are always risks...)
There is an NOS kit with that set-up as well as a Zex kit. Both would do fine...
I'm pretty sure that both NOS kits would come with everything needed to use the kits...
(You might want to consider a blow off valve and a nitrous pressure guage)
------------------
"The Yellowjacket"
1998 Mustang GT
13.97@103
"Hey, you weren't supposed to be that fast!"- Turbo Eclipse Driver