This is what I had to do to mine. Unless you want shavings all in your coolant system, you need to pull the crossover off. When you do, you may wish you had bought a new intake if you got one secondhand. I would never recommend buying a used PI intake after seeing what years of heat cycles does to the plastic underneath the tstat.
Drill bit is 9/16 btw, you don't go right to that, work up to it in stages. Tap is a 3/8 -18 NPT which you can get cheap from Harbor Freight. I also highly HIGHLY suggest using a drill press.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.