I would reckon heavier hammer spring if you plan on running pressures higher than 850 psi (co2 average at room temperature). The VERY FIRST thing I would consider besides the springs and doodads is making sure that valve stays in the tube under pressure. We talk a lot about modding these guns, and a lot about how to make them faster, stronger, better than before....but it wont matter a bit if the valve is improperly pinned into the tube and you end up wearing it inside your chest. Ok, I'm done with my soapbox! Have you asked the guy who is making you the tube what he recommends as far as springs and such? If this person is making PCP QB conversions I would guess he has already figured this stuff out. Just a thought.
Be safe,