When Carl Moesly designed the 19/20' hulls in the mid 60's, the biggest outboards (I-6 Mercs and V-4 OMCs) weighed about 300 lbs! As a veteran pilot and race boat builder and driver, he was well aware of the importance of CG (center of gravity) location, and even patented a ballast tank design (see
Moesly Seacraft website) that he used in the race boats to adjust the CG location for running into either head seas or down sea. He balanced the production boats with the fuel tank centered right on the CG so you wouldn't change trim as you burn off fuel. You can be sure he put the CG where it is to give the best all around ride and performance with motors that weighed about 300 lbs! As motor weight goes above that, you're getting progressively further from optimum balance and performance.
The 20' SeaCrafts have relatively narrow hulls compared to newer boats, which is one reason they ride so well, but you can't shift the CG aft a bunch by hanging a 500 lb motor on the transom and expect them to perform like they were designed to! I doubt it'll be self-bailing with that much weight on the transom unless deck has been raised. A lot of guys run the smaller V-6 carb'd Mercs on stern-heavy cc's and seem pretty happy with them, so 350-400 lbs is probably tolerable, but I'd sure think twice about 500 lbs, as that's way more power than needed anyway unless you just want to race the boat in flat water! A stern heavy boat will tend to run bow high, will be hard to get on plane, will have a high min-planning speed (a big deal in rough water!), and will not ride as well in rough seas. You can compensate with trim tabs, a Doelfin and a 4B prop, but why start out with a non-optimum combination if you don't have to!