Why are 318's low HP and torque?

I already have a 318 engine from the 80's so I must have the 302 heads already right? I looked at them and they have an "M" stamped into them....? Hmmm. Don't know durrr.

Oh yeah I have the stock 7 1/4 rear end in there right now. I am looking for a 83/4 rear end while I get the engine and trans in order....