Use a simulator to work out things like the hanger length, arm ratio, sling length etc.
Looks like your mass ratio is ~37.5:1. Thats pretty low (Normal is ~100:1, which would mean 3 pound ammo here), so I'd guess, especially given that you have wheels, that you will get better results with a fixed counterweight than a hinged one. Either way, you will want a pretty low arm ratio, maybe even as low as 2:1 ( A simulator should help greatly here).
Also, personally I've found fixed counterweight trebuchets simpler to tune (and to work better when poorly tuned), but its been a long time since I worked with either type.
Anyway, I strongly recommend checking out a simulator of some kind. It will let you get a good sense of what dimensions to choose, and what adjustments have what effects.
Sadly I haven't kept this site very up to date, so there might be some newer/better simulators around. If you find any, feel free to add them to the simulators page.
Lastly, its mostly just me here. You will get lots more great advice at the The Hurl Message Board. Especially if you want structural advice, you will get much better help there, since I haven't built anything very strong at that scale, and many of them have.
Good luck, and stay safe! A treb that size can be pretty dangerous (Remember, you are very likely to throw backwards, or trigger unexpectedly for example!)
