The only problem with internal wastegates is sometimes they are too small. So lets say you want 6 psi, when the wastegate fully opens not enough exhaust gas bypasses the turbine (because a 351 is pushing a large volume of exhaust) so what happens is your boost creeps up higher then you want maybe 10-12 psi. Normally not a big deal, but because you only want 450hp you are going to have to run a very low psi to keep your power down. So if your boost is creeping up to 10-12psi that could easily be making 500-600hp and broken transaxles.