Sherlock, welcome to CHR. I see you're from Denmark, which hopefully will guide some of the replies to be more towards the simple facts. The easiest way to really check the horsepower output on your engine is a chassis dyno that measures rear wheel horsepower, but finding one in Denmark may be a challenge.

If it's indeed a low HP 350 there's no "bolt on's" that will give you huge boosts in power. A 3" exhaust would be a mistake - too big, and it won't scavenge as well because the flow velocity will be too low. If you're on a budget and simply wanting to get a little more power you can buy a set of good, aftermarket headers and a new intake & carb package that will get your engine breathing better. If you're wanting to go bigger, then you're faced with changing heads and cam. The problem is you really need to decide on your plan before you start spending money or you may buy things that you'll only replace later.

Hopefully one of the guys who know far more than I do will chime in here, like Glenn Sexton or TechInspector1, and provide you with some of their wisdom in detail. I know Glenn recently posted details of a SBC build that's tried and true, from bare block to performer.

Good luck with your project. I've been amazed by some of the cars coming out of Denmark in recent years! Some fantastic engineering, and attention to detail.