I have designed a number of jib cranes, gantries and similar over the years. There are a few major factors to consider here: foremost to me is the Stability of the crane. This starts at the base- the type of truck chassis it is mounted on and the type outriggers used are the determining factor here. You do not say what type body is on the truck. With a light truck, a "mechanic's body" (toolbox type body) and a jib, IMO, you should design a set of outriggers and locate the jib at the junction between the back of the body & the rear bumper. This gives the greatest distance from the center of jib to the center-of-gravity of the truck. That is what is going to keep the truck's front axle ont he grund when you start picking loads.
Preliminary Design of the crane can be a kind of bass-ackwards affair. You have a truck, you know its dimensions and weight. It is a very light-duty truck, so things are going to be critical in terms of stability and flexing the frame of the truck vs. jib capacity. Possibly you can get it to a truck scale and weigh the front and rear axles spearately if possible. This will give you the weight distribution. Figure where you want to situate the crane and figure the approximate radius from the center of the jib out to the hook. Figure the "countering moment" made by the engine, cab, body and any other equipment on the truck acting about the center of the jib. Consider the outriggers of the jib as planted on solid ground. In effect, the load on the hook of the crane creates a moment which wants to lift the truck by pivoting it about the outriggers/center of jib. The countering moment = the weight of the truck (you will need to get the center of gravity of the truck based on engine, cab, etc) x distance from C.G. of the truck to the center of the outriggers/center of jib. This will give you a moment in foot-pounds. Now you have to decide which is more important to you: available lifting capacity or distance from the center of jib. If you pick a load, divide the countering moment by the load you need to pick and you will have the allowable radius of your jib.
OK, now you have determined what capacity you can pick and at what radius. This is predicated on your having a solid, straight load path from the post of the jib down into the outriggers, and the outriggers on solid ground. You are transferring the load from the post of your jib straight down intot he outriggers, and relying ont he weight of the truck to act as a counterweight to the load out on the jib. You have established loads and distances, so you can then design the structural members. Start with the jib boom, then the post, then the beam which will transfer the load to the outriggers. Add bracingin the form of gussets.
A few details: Power is typically a 12 volt winch for the main hoist on small jibs. If you want to get fancy, you can arrange for the jib boom to riase/lower using a hydraulic cylinder, similar to an engine hoist. On a small jib, if you do add a boom lift, you could use an engine hoist type cylinder and manual jack pump. Or, you can use a 12 volt snowplow type hydraulic pump with solenoid valves. You will need a pendant type controller for the winch and boom lift (if you power the boom lift).
You have a very light truck, so can;t build too heavy a jib for it. I would not recommend you have the hoist rolling on the jib boom (arm).
Design a positive lock on the swivel for the jib on its post. A clamping mechanism is good as it can hold the boom and put varying friction on the swivel so you can turn the load manually and slowly. Provide a positive lock with a pin for travelling with the truck- that way, the boom can't swing in transit.
To lock things for travel: mount an eyebolt- use a forged steel shouldered eyebolt- on some solid part of the truck body. Locate the eyebolt so it is under the hook of the jib and the lines are plumb or close to it. Fishplate the eyebolt so it does not pull out. Put a shackle in the eyebolt that can accomodate the hok of the hoist. To travel with the truck, you simply hook onto the eyebolt and take strain ont he hoistline with the winch.
Hope this helps- call it the ramblings of an oldtime engineer.
Joe Michaels