Bleeddodge, you're at it again! The '73 455SD was rated at 310 NET horsepower, which translates to about 400 GBHP. No 318 Dodge was EVER produced as a "performance" engine. The 273 and 340 carried that job for the baby Mopes until '73, when Dodge learned 360 heads would fit 340s, making them smog turds, too.
In reality, the 455SD was the very last "muscle car" engine made by Detroit, though it never found its way into a muscle car. It was scheduled for the '73 GTO and Grand Am, but never materialized. There is one infamous photo of a '73 GA with the SD badges. There's still a debate today, whether or not there was REALLY an SD in the car.
As for the potential of the SD, change the cam to the 041 (factory muscle grind) and bump the compression to 10:1, and instant 500 horses! They had the best flowing heads (aside from Ram Air V) of any traditional Pontiac V8. NEVER underestimate the GM engineers when it comes to circumventing the system! Especially Pontiac, who carried the banner for performance through the dark ages. In '79, you couldn't buy a Dodge OR a Ford that would turn the tires over from a stand. Trans Am would...even with low compression and the obligitory automatic transmission.
One last thing. Due to EPA and other government rules, compression ratios fell through the floor in '71. ALL American performance cars suffered from then on.