First, Welcome!
2nd, to your question.
300HP depends on where you want the HP? 300HP at the flywheel (like modern cars are rated) is not too extreme. Engine modification will likely include a new AL head, carb, probably either really high compression or forced air of some sort. New Cam, ignition, headers would also be needed. That base package is around $3000 or so, depending on how you source the parts.
Then, however you have to factor in other things needed to accomodate that HP. If you have the generic 6 cylinder transmission (either 2.77 3 speed manual or C4 auto) you will likely have to upgrade as that much HP will likely chew it up fast (at least the 2.77, not sure on the auto side). That much HP will also eat up the stock rear end. I'm around 200 BHP on my car and that chewed my stock rear end up in a matter of months. I upgraded to a V-8 8" rear end to accomodate the power. That means new brakes in the rear as well. That will also mean you will likely want to upgrade your front brakes so that your back brakes are not larger then your front ones, so you have matching lug patterns (though there are some 4 lug 8" rears around), and also to slow you down since you have more power. I would say all that will run you anywhere between $800-$ 2000 depending on choices you make.
Just wanted to put it in perspective so you wouldn't be surprised when you started digging into the projects.