Will,
I guess it goes without saying that an increase of 200HP to 300HP isn't going to be achieved with a couple of bolt on goodies, other than perhaps a turbocharger!
The good news is that the engine can produce a hell of a lot more power than 300HP, so it's just a matter of how you achieve it. Where do you stop? What gives the biggest "bang for buck"?
To my mind you want more torque and power within the same rev range, then you don't have to worry about the crankshaft and things like roller cams. What you consider doing to the engine depends on what condition it is in. The bottom end doesn't need much doing to it at all as the stock crankshafts were very strong to begin with.
Improved breathing is a must. A decent increase in power and torque could be achieved with some mild porting of the heads, bigger valves, a better intake manifold, and a modern carby with tuned jetting. Ideally you should also install some different pistons to increase the CR and a higher performance cam shaft to take full advantage of better breathing.
I'd also go for an electric fuel pump with pressure regulator, it could also be worth replacing the original water pump with an electric one.
There are a few books on the specific topic of tuning Chrysler "A" Series Smallblock engines.
OR you could just buy one of these
Crate Engine - Chrysler 360 Engine