What engine would best fit my car?
The easiest solution is to sell the V6 car and purchase a Mustang with a V8 already in it. By the time you spend the money on the parts, and pay for any labor for things you can't do, you would have spent more than you would if you just got another car. Then you can "hop-up" the V8 car and spend less money in the process. If you prefer to keep the car you have, then purchasing a wrecked V8 car would be the way to go and stick with the same year if possible.