I prefer shocks straight up. That means that no matter where the suspension is, you're still getting the same damping. If you install a shock at 45* and double its valve damping, it offers the same damping as a straight shock... but only at ride height. As the axle raises, the angle gets steeper and it provides less damping. As the axle drops, the angle gets less and provides more damping.
The common application for angled shocks is in racing applications where there is little or no suspension deflection at all. In that case you don't need to worry about that damping angle changing all the time. In those cases, the shock is installed on a parallel plane that corresponds to a line drawn between the lower control arm pivot point and the roll center of the car. That way the forces applied to the shock are directly countering body roll on the same plane.
To determine length, take the springs out and rest the axle down on the bumpstops. Measure the distance between the upper and lower shock mounts. Subtract 1". That is the minimum length the shock needs to compress to. You want the bumpstops to stop the travel, not the shock. Then re-install the springs and let the axle hang at its lowest point. If its coil springs, let it drop to the point where the springs are just loose enough to turn with a little resistance, but not fall out. Measure again to the mounts and that's your extended or maximum length. If its a leaf spring axle get your max length this way: With the springs installed, jack up one wheel as far as you can until the unjacked wheel comes off the ground. This is best done at final weight. Now measure the distance between the mounts on the unjacked side. This not only allows for full droop, but full articulation when one side is up on curb or something.
Always err on the side of shorter for both measurements. If you make its minimum length too long, it will bottom the shock before it hits the bumpstops damaging the shock. If you make the max length too long you run the risk of letting the axle droop too far which can cause a coil spring to fall out, or the driveshaft to pull out of the tranny.
If you don't already have mounts installed, you have a world of possibilities. The main measurement you want is the suspension throw. If you measured 12" at the shortest and 19 at the longest, you have a 7" throw. Since you don't have the mounts already in place, you can choose any shock with a 7" throw and mount the shock studs anywhere.
When it comes to applications, valve damping is widely variable. You might find a shock that has the right throw, but was standard on 3/4 ton trucks. Fine for heavy loads, but not wise for handling or ride quality. They have way too much jounce damping and not much for rebound. Conversely, sports car shocks have tight valving which may not work will with the fatter tires you probably will have on there.
Be realistic when you get to that point. You didn't mention what the you're doing with this truck which would have helped me narrow this down and give you specific answers.