If I remember correctly when I came across that 300hp crate motor it had the rt+10 cam and 2.02 intake valve heads when combined with the testing without accessories as mentioned above easy 300hp without much effort.

 

Here's a direct quote from that hot rod article mentioned above:

"Mopar’s Magnum 360 crate engines were offered in 300- and 380hp versions, both based on the production Magnum 5.9. The higher-powered engine comes with higher-compression pistons, a long-duration camshaft, and a single-plane Mopar Performance intake. Far more basic, the 300hp version is the same engine as in your typical Durango, van, or Ram truck. With the 300hp crate you get a bone-stock production bottom end, complete with the stock truck pistons and cam. Changes are only the addition of a center-sump passenger-car oil pan, an LA engine-style front cover, a conventional distributor, and a two-plane carbureted intake to replace the factory “beer-barrel” MPI injection system. The fact that our dyno shop had one of these stock-based crates sitting around gathering dust sealed the deal. Even though our engine began life as a crate motor, our results would be representative of an effort that starts with any serviceable or remanned production 5.9 Magnum."

So it was not the same intake set up and from my personal experience with a dual plane hughes the factory manifold sucks above 3500rpm. So the better intake coupled with who know what higher flowing carb is what my guess where the extra power came from. It was the 380hp version that I must've read about with the better head and cam.

Link to article for anyone interested, its a cool article. http://www.hotrod.com/articles/hrdp-...-mopar-engine/