4.11s but no power

[Mertz]

I finally got around to checking my gear ratio on my 72 C20 and found the tag that told me it is a 4.11 Dana 60 non posi. I don't think the cover has ever been pulled to I have to believe it is correct. With my t350 w/ shift kit and tuned 350 getting probably 290hp I still have problems getting off the line and I can not burn the tires. I am running 265/75/R16 in the rear. Where is all my power going? I should be able to break them loose at any time. Could it be the torque converter? It was new when the trans was rebuilt a couple of years ago.