Depending on the level of the build, I think it is mostly a wash. The only bennifit I see in the use of a Magnum engine is on a low or mild performance street engine.
Reason?
Newer engine with less heat cycles. 1.6 rocker ratio. Hyd. roller cam friendly. Easy HP made. MoPar create engines are a good example. 360/380HP engines were a stock engine upgraded with cam, springs, intake. Given a carb and headers, they constantly dynoed over 400HP with stock head ports.
Not to shabby IMO!
Heavy street hitter/racer? It's a wash. Mostly.
A bunch depends on what equipment your going to use.
