I've done quite a bit of researched for upgrades to the 6.2 and I'm sorry to have to report that there is no inexpensive way to get the power you want.
I have a pair of 6.2's in my Formula 312. After my stock exhaust manifolds rusted out I also replaced them with Stainless Marine manifolds and I lost horsepower!
The 6.2 is a great engine, for the power range. But all the components are designed to that one specific power setting of about 320 HP. The Stainless Marine manifolds flow more exhaust, but in this engine, if its not coupled to improvements in the intake manifold and the heads, then the exhaust is sub-optimized and actually hurts performance.
If you are trying to upgrade the 6.2, the best option I found was to bolt on a Whipple super charge. That was what I was thinking when I replaced the stock exhaust manifolds with the SM ones.
But if you do the math, with all the stuff you have to do to the 6.2 you are probably better off just selling the engine and buying a 496 HO or 502HO. Also, you can boost the power of the 6.2 but you will be stressing an engine that wasn't designed for really high power.