What cam do you have in your magnum?
I have an Oregon #1357 in mine and love it! NA with a 770 (a 670 would probably be better), 727, 3500 stall and 4.30 gears, stock bottom end and off the shelf EQ 2.02 heads my car ran a best of a 7.60 in the 1/8th. That's low 12s, high 11s or something in the 1/4. I'm running the same combo now but boosted.