If you want a V8, sell your V6 and buy a V8. Not trying to be an *** or a downer or anything but you'll save money that way. You will need the trans out of the Marquis as well and then you'll need to swap the tailshaft housing with a Mustang one since the Mustang ones are shorter. Also going to need fuel, harness, ECU etc... It's not just a drop in. I mean it can be done but if you don't have experience pulling motors and re-wiring cars...
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.