98 vs 99 Nitrous kit???

I can find used 99+ nitrous kits alot easier than 98's. My question is what is the difference between the kits and why couldn't a 99 kit fit on a 98?? They're both 4.6 engines and whatnot. Is a zex kit universal basically, one that was on a 5.0 will fit on a 4.6? I'm sorry for the redundant questions. I searched but didn't know quite what to search for. I should have posted in power adders, but this section has alot more people viewing, so i thought i'd get a faster response. Thankyou
 
If you're looking at wet kits it makes no difference what so ever. All years take fuel from the fuel solinoid from the schrader valve. So yes, your fuel system is different than the 99, it's not entirely a returnless system, but not completly like the 97 return style. If you want a wet kit, any year will work.

If you want a dry shot, it depends on the kit.

BTW- just get a NX EFI wet kit, it's the best and safest :)

Just saw that your sig mentioned a ZEX kit. Those are pretty good and up there with NX quality. The solinoids aren't quite as large as NX and can't support over 125 hp, which doesn't matter for 99% of us.

Anyway, is the ZEX kit wet or dry?