For my Dodge these make the most power Flowmaster’s retrofit-style 4-2-1 Scavenger Series. I built several sets of headers for the car, used standard 4 into one and merge type collectors. 512 wedge NSS type car. There is a program called pipemax that allows you to size your headers correctly, and give you lengths for step headers as well. http://www.maxracesoftware.com/
It also has a pretty good engine output estimator in the program as well.