How much power will a ats stage 3 hold
ats told me know more than 500 horse
kyle michael told me he ran a stage 3 for a year with no problem at all
he was at or around 600 horse

I'd say if ATS says no more than 500rwhp then that's where they are comfortable with it. Anything above that may last or may not last over a number of years depending on how the driver drives the truck. If you are looking to overpower the StageIII then get a Stage IV. You can never have enough tranny for these trucks when you are increasing the power IMHO. I went with a SCIV vs a SCIII because I'm not sure how much power I'll end up with in the future and the price difference wasn't that much.

I read IIRC 550 hp 3 no clutches but co-pilot and 4 clutches but no co-pilot but 5 clutches and co-pilot jumps to 700hp.

:exactly:I couldn't agree more with Lakingslayer...I've personally had to go back and re-do mods because I didn't go big enough the first time around. My motto now is "Always go more that you think is adequate". You'll seldom be sorry! :)