1972 GMC super custom

[Joeyfish]

Okay, so I bought a running and driving 1972 GMC Super Custom 2500 truck for $500 a while back, and I have some questions.
1. The guy said that there was an oil pressure problem, but didn't know what could be causing it. The gauge says it is always high whenever I start it but I don't know if it could be a faulty gauge or just whatever. Is there any way I can check it to make sure everything is correct? It doesn't smoke, it runs pretty smooth in my opinion, but I am only 17 so what do I know. Is this something Autozone could check for me?

2. I am a teenager so of course I want a loud sound system in my truck. The gas tank is behind the seat, and I don't know if a sub woofer will fit under the seat either. Any places I could maybe put a sub or two?

3. I want to lift the truck a little bit (maybe 2 or 3 inches) and I don't have enough money for a lift kit. Any cheap ways to lift it? I am very good at improvising things to make it work, and was just wondering if there are any "ghetto" ways to lift a truck that would be free or fairly cheap.

This is my first post so I apologize ahead of time if I put this in the wrong spot or whatever. Thank you. -Joey Fish