Did I install the wrong oil pump?

Unfortunayely, when you run a high volume oil pump you should be using a larger oil pan. This is why it didnt fit. With your setup, you shouldve just stuck with a stock oil pump unless you planned on going all the way. windage tray, large oil pan, etc. I have also heard many of times about the high volume pump running a stock oil pan dry at high rpm. I cant confirm any mishaps, but have been told that many times by reputable people as well. Out of curiousty, who told you that it was a myth?