Don't notch pistons... been there, done that. Just get a complete running Explorer motor pullout. Upgrade valve springs, change out cam for the GT cam, swap any accessories needed from your car to the exploder motor and drop it in.
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 ****.