I got around 800 hours on my 421 without a rebuild. What I did was ran klotz super tech @ 32:1, I did 5 heat cycles and for 1 tank of gas I just drove down the road going up to the power band, lightly getting into the power band for 4 gears and then I would hammer into it for about 5 seconds and then back off of it. I did this for about 20 min at a time and then let it cool off for about 30 min. I did this process for 1 full tank of gas and it worked great for me.
But as stated before, there are many methods for break in, that's just how I was taught