I don't run any MSD stuff anymore but I would buy the baddest box you can afford. Most power adder stuff grows to more and more power and you might as well avoid buying twice.
I use the Leash single stage board and it works great! It made wiring up the nitrous system super simple.
'63 Dodge 330 11.19 @ 121 mph Pump gas, n/a, through the mufflers on street tires with 3.54's. 3,600 lbs. 10.01 @ 133mph with a 250 shot of nitrous an a splash of race gas. 1.36 60 ft. 3,700 lbs.
