2 of my favorite topics, 750DP's and 509 cams. You should have enough gear and conert for the 509, now how about compression? If you don't have 10:1 it's not going to work well. The vacuum carb is terrible for a low compression 509 cammed car. (been there and it ran like poo poo) A DP will help but you're going to have to tune it. A 750 DP is way beeter IMO than the vacuum modle. What else is done to the motor? headers? intake?
FWIW my old 440 w/ 509 liked 38* total at 2400 no vac advance and you won't really feel it pull until 3k then watch out.