I echo what everyone else is saying here. 300-350hp
at the rear wheels is going to be a tall order for your engine. is it possible to build a 350P to make that kind of power, yes. But it's not a bolt-on endeavor.
With the engine you have, you've got to contend with very low compression, cast pistons that won't hold up to much abuse, cast rods that won't want to spin as fast as you'd need this engine to and an oil pump that isn't really up to the task either.
In it's stock build configuration, doing a 4bl swap, with a small cam upgrade like a factory 067 cam or maybe something like a Comp 250H and headers with an upgraded exhaust is probably the limit. You're probably talking somewhere around 275 crank hp.
Now, going to a more performance oriented rear gear is also going to help you out a bit here, and the torque the engine makes will make it feel like it has a bit more pep.
If your goal is 350 to the ground, streetable and reliable, I like others agree that a swap to a 455 or stroked 400 is a good option. Depending on what your budget is, I'd probably go to somebody like Len Williams and have a basic 455 made. He markets a 425hp stroked 400 that is a nice piece and will get you to your 300-350 wheel hp target.
Or, if you want to get a little hairy and mess around with your reliability you always have this option.
https://www.youtube.com/watch?v=izBrYc7LI7Y
I doubt an engine like the one referenced in the YouTube video above would hold up for very long. But it would be cool while it held together.