I have built several headers using pipemax software, it works really well. If you have the room and take the time, I see improvements by building the step headers. I like fenderwell style headers because it makes stuff easy to work on, so I just choose a longer harmonic, and again it always makes good power. I personally have had good luck on my drag car with the old style Hedman 4 into 2 into 1 style collectors which make more low rpm rang torque. My racing buddies made fun of my old school collectors so I tried some of the merge style and the car slowed down. I would think on a land speed car you would want just a good 4 into 1 standard style collector. And on the projected HP numbers, most of the engines I build come up at the average HP number generated by pipemax. I also use performance trends engine software and it hits within 10% error when I dyno the engines.