Diesel Dart Cummins

Pull the timing to the head (as people say) it really compliments added fuel. The 4200 spring does not mean it will turn 4200 rpm, I can almost promise you will run out of fuel supply before that especially if you are on a stock style lift pump.
I used a Holley blue pump to supply the injection pump, the most I saw was 4100rpm on my tach once. Pump tuning is great on these things, I had my screw about one turn from all the way in, it tried to run away a little bit if it was all the way in lol.
Timing spacer... never ran one, I wasn't convinced it would do enough for me, read too many mixed reviews on them.

Just to be clear I'm not telling you to do anything I haven't done, I had a 3200 spring for a short time but I still felt like I was hitting a wall, added rpm and WOW!! Truck flat out ran!