Not to sound negative, but I doubt fabbing a intake would be of much help. The heads have to be able to flow, they, with the cam are the heart of making power and along with them goes compression etc.
These brute engines use very heavy pistons and the crank is also heavy. They were made for pretty much torque use. But, torque is what moves the vehicle, and some diesels are indeed very fast even so they too use rather heavy pieces. You could likely go in and see if there`s any flow gains to be had by porting the heads but there likely isn`t a larger cam for this engine unless you have it custom ground ($$$) Next you can likely fabricate a set of headers for it also which would be of help. But the limiting factor is, your trying to hot rod a engine that wasn`t intended to be a hot rod. Not saying it can`t be done though.