Magnum heads on an LA enigine

Your not exactly comparing apples to apples there when comparing a stock 2 barrel LA 318 to a Magnum headed 318 with a (I'm guessing) 4 barrel carb, better intake, much more aggressive cam, headers and good flowing exhaust. Possibly higher compression also. Desktop dyno is only as good as the information input to it and even then it's not perfect. In other words a bunch of things can affect it's output and it's usually on the order of 5% higher than you'll actually get. To make the comparison apples to apples you'd need to use the same cam (except add a little more lift cause magnum rockers are 1.6 ratio), carb and intake, headers and exhaust but change just the heads and see what happens. You also need very reliable flow numbers for each head and input the correct valve sizes for each on not only the main screen but on the head flow file.