shaker scoop on 98gt?

You might want to check out Banshee Performance. They include their own custom hood with their shaker kit, and have kits for Mustangs going back to the 1986 model year. For the 1998 model year, check out their FAQ page.

However, I don't think any shaker will work with your stock GT hood because of those existing "Maverick Grabber" style scoops.
 
http://www.classicdesignconcepts.com/tech_center/shaker_96-98_installation.html

This shows the installation, it seems like a lot, but it took me 30 minutes, and most of that was time taken to ask myself why I was cutting a hole in a perfectly good hood. I love my kit and it looks different enough from the Mach1s that I dont feel like I'm trying to be one (yes I'm borrowing styling cues). Any how its a really good kit, I dont know if you have to buy a hood for the 96-98 go to the CDC site and drop an email to Kim or Marnie, they are both extremely friendly and helpful.

I'd choose this over the Banshee kit just IMO I dont think the Banshee airbox looks right since the release of the 03/04 Machs. Good luck with whatever you do, a shaker on a pre 99 will definately stand out :)