Jim, Welcome to the Hurling Forums!
No worries on the mis-post. I'll nix the one in the Administrative Section since you re-posted it here and we can keep responses in one spot for you.
To answer your questions, sure it sounds doable. Since you are mounting to a trailer, your height is only constrained by any bridges you need to go under (and keep it under 13' 6" road height to stay legal regardless). You'll want to dis-mount the throwing arm when traveling so keep in mind a way to do that. So an 8-10' high main pivot (from the trailer deck) will be optimal use of your space to shoot a pumpkin.
Standard hinged counterweight trebuchets (HCWs) like you are looking to do usually use roughly 100x the projectile weight for counterweight, so up to ~1000 pounds in your case. That's a lot of weight, but not crazy given the size of materials you are looking to make your frame from. That ought to get you well out past your goal, less weight will shoot shorter, but likely will still get you out to 500 feet after much tuning work. Less weight is easier to cock (I highly advise a winch or ratchet or similar for this!) Steel or aluminum frame, the choice is up to you, but at least the arm I would suggest aluminum to keep it light (it needs to turn as fast as possible). I personally find steel easier to weld than aluminum, and it grinds better, but if you have experience with both than just go with whatever you prefer. Steel can be painted to minimize rust issues.
The most important part will be to keep your axles' lengths to a minimum, especially the main pivot. Braces extending inside the frame to support the axle just to either side of the arm will help greatly in this.
You'll want an arm ratio (long side to short side) in the 4:1 - 5:1 range, and the arm should be long enough that the tip of the arm when cocked is practically on the ground (or trailer base) and forms a 45-50 degree angle with the ground. Simple trig will help you out there with the exact sizes. As a starting point, the sling should be about 75% of the length of the long side of the arm, the exact length will need to be tweaked as you tune but that's a fair starting point given the parameters I've spelled out above.
- Slinged Treb Still.png (9.02 KiB) Viewed 14456 times
We can get to tuning once you've got it built should you want some help there.
More questions? Just ask! Some more folks will pop up to help you out too. Post pictures as you build, we love pictures!