Cheapest way to make 300 crank hp from a 5.2 magnum

If you do not have power steering, you can use the Dakota manifolds with the 2.125" opening. They were only available in the early 90s but also in the 1500 trucks as well. I think with those manifolds you'll be spot on the 300 mark with everything else you've mentioned. The magnum heads are good pieces. With headers, it would be a no brainer for 300 HP.