In order to help, we`ll need more information. What kind of truck are you thinking of doing? A box truck? a 4x4...etc? Do you want to make it out of just buttercream, or will you be using fondant to cover it? etc
You have plenty of time to plan ahead for this one. THe first thing is to determine how many servings. Next, make/find a pattern for whatever type truck you need. Draw it all out on parchment or paper. You'll need a side view and a top view. Once you make a side view template, I would make cardboard template. After you bake the cakes (I would start off with loaf shaped cakes), you'll hold the template up against both sides of cake and follow that to carve the truck shape. For the front, use the flexible, paper pattern (top view) to guide you to make the front of the truck. Repeat for the back. This is really the toughest part.
Now, assess what can be done ahead of time: tires, runners, exhaust, lights, etc. Make those from gumpaste and after they dry, place them in plastic container.
Also, something that can be done in advance is make a platform for the truck body to sit on. It can be as easy as foam board with clear plastic 3" pillers glued on. For this base, just follow the top view pattern.
Sorry so long - aren't you glad you asked? 
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