If you put a well built 351 in the car, it would have more power than a 289 with the same mods. To put a 351 in your horse, you have to mod the spring towers for the exhaust to clear, headers or stock manifolds. Not a big deal, but do you want to cut the towers on a stock Mustang??? Your car, your decision. A 289 that is put together with the right cam, heads, ignition, and carburation will be a very strong motor. You won't even come close to putting a good stroker motor together and be within your budget.