It's probably got cast builder pistons with a giant dish and poor deck height putting it in the 9:1 compression range. The two barrel is probably the biggest limit right now. I'd guess in the 250HP range with 480lb-ft of torque. Putting a decent dual plane 4 barrel intake (Performer, factory cast iron), and a decent 4 barrel Holley (750cfm vacuum) would probably gain you 20HP. If you have cast exhaust manifolds, pitch them for a cheap set of headers as well.
You're going to need a lot more of everything if you want 500HP. You can build 500HP with the shortblock, but it's probably not recommended if you want it to live. Fully port the heads, a lot more cam, bigger intake and carb, etc.