Oh shit, here comes the "I am faster with less displacement" arguement again.

My advice is to do whatever the hell you want to do and not let anyone tell you what you should do. If you want to keep the Mustang, we can help. If you want a 240, we can help. Make the decision for yourself. Personally, I'd like to see the Mustang fully restored as most folks don't bother with them unless they are decked out in the Pace Car regalia. A fox body is but a blank canvas, plan before you paint and you will have a masterpiece.