This is a very common question that people ask all the time. There is tons of valuable and great info on this site regarding this very question. I went through this about 4-5 years back with a bone stock merc 454/330hp. If you click on my name a drop down will appear letting you been my previous post. If you go back to near my first post and scroll through the titles you start seeing post about getting more power from a stock 454. I asked a lot of questions, got a lot of good info and posted updates on all the upgrades I did. When it was done I had changed the heads to large oval ports with closed chambers and stainless steel valves, a crane hyd flat tapped cam, roller rockers, I ditched the circulating pump for a cross over, installed comp cams pushrods, a Edelbrock performer rpm intake (not the air gap), a marine holley 750 double pumper, a K&N flame arrestor, stainless marine exhaust, a Cloyes true roller timing chain, Msd marine distributor, marine ignition box, marine rev control box and Taylor plug wires.I found parts on eBay, OSO, etc.... Most of these parts you can find used at 1/3-1/2 the cost of new. I got lucky on several parts like the carb (it was almost brand new), the msd box & rev limited were new (I found them on eBay new for $180.00). The roller rockers, pushrods, exhaust and distributor were used also and picked up at decent prices. Msd will also completely rebuild your distributor (with new cap, rotor, pick up, etc.... For around $140.00 (I had mine done 2 winners ago... it came back looking brand new!). I also picked up a set of Teague Marine valve covers on eBay for a great price. You just need to shop and look as often as you can because the great and good deals are jumped on quick! If you need any help feel free to message me your number. I'd be more than happy to point you in the right direction or answer any questions you might have. I run a blown 516cu bbc now with a M3 Procharger that pushes around 850hp. I've just keep building and changing things! Lol But for the most part, I've done very close to what you were asking about in your post. Good luck! Mark