There are no "rules" to building a proper front suspension. There are many factors involved such as :arm length, arm angles, caster angle, camber angle, king pin inclination and so on. There are however ranges that for those values that are considered "normal" or good to stay within. I cant tell you that you should make your upper arms X inches and your lowers Y inches, those lengths are dependent on so many other things.
To help answer a few of your questions:
The lower arms don't necessarily have to be longer than the uppers. However the advantage to having longer lower arms is that the tires gain negative camber when the suspension is compressed. This helps with traction in cornering, especially under speed. The angle at which the arms are to each other affects the cart's roll center or the amount of body roll.
The best shock angle is the one that closest follows the suspensions path of travel and uses the shocks entire effective stroke without over extending it or bottoming it out. Because of this I would reconmend building the suspension first to determine the travel of the shock you need and not building a suspension around a shock you already have.
As a rule of thumb it's better to have more distance between the arms. At the spindle end of the arms, this is determined by the spindle height, which is usually limited by the diameter of the wheel. So I'd say as far as your wheels will allow you to.
You can use heims or bushings at the frame pivot points, both have their ups and downs:
Bushings (I'd reconmend derlin) are simple, will last longer and are quieter. With bushings you must build the geometry correctly because it's "built in" and can't be adjusted.
Heims can be noisy and they don't last forever, but they offer alot of adjustability. You can even change your caster angle by moving either the front or rear piont in or out which causes the point at the spindle to move forward or back.
Tubing size? I'm currently building a custom suspension using 1 1/4" - .095 wall tubing. This is the same size Todwil tells me he used. The easiest way to attach heims is to use premade bungs that slide into the end of the tube and get welded around the perimeter. They are drilled and tapped to accept the threads on the heim's shank. To match the heim size, once you've determined the tube diameter you're pretty much limed to a few sizes. For 1 1/4" there are bungs available for 1/2", 5/8" & 3/4"....I'm using 5/8". What bungs are available also depends on the wall thickness of the tube.
I'm currently hung up on weather to use derlin bushings or heims at the frame...I think I'm back to heims, because I like the adjustability and hopefully my engine will be loud enough overcome the noise
I know that doesn't give you exact answers, but I hope it helps. I'll try to answer any other specifics I can.
Mike
