Cheapest way to make 300 crank hp from a 5.2 magnum

I'm sure you want a pump gas motor. At 10:1 you won't have one, not even with Magnum heads. Be careful there. Most I'd go would 9.2. The .8 difference won't be worth it in power and you'll have an engine that needs to be spiked with race gas or octane booster.