Exhaust limits/Carb choice

Sounds like it will be a fun combination! Repair the headers and use them, since you've already gotten them with the car. Either carb would work good, once tuned correctly to the combination. I run a 600 Holley DP on a 318 with a 904 trans and a small converter, so I can say from personal experience that a DP and a auto trans can work well together.