I love this idea. I agree with WD about finding some parts in urban areas where we already find certain rare things. I propose this outline about the "Build Your Big Truck" quest, which is activated once you [somehow] get either the chasis or the motor back to a tent or base:
Cab/ChasisTough to find: 7/10
Obtain a salvageable cab/chasis either by finding it in a rare urban random encounter, or by buying one when they are [rarely] available at certain vendors: the tanker in SF, that weird robed guy next to the Bazar at NCR, or some of the outdoor vendors who we normally ignore. And then, only through a dialogue option that you simply cannot see if you don't have this quest. Price: high
Motor/TransmissionTough to find: 10/10
Obtain a salvageable motor/tanny...same as above. Price: very high
Requires: parts! parts! parts! + high repair skill + manuals



WheelsTough to find: 3/10
Obtain from T-Ray of course! But only when he has them, which is not often. Special dialogue enabled via quest. Price: varies.
ElectricalsRequires: Science 100
Craftable:
BedRequires: Repair 40
Craftable: 36 wood + 18 metal parts
BodyCraftable: 36 junk + 24 metal parts
Other Requirements: Armorer 2
Sorry had to go - will edit this post later!