well yeah, the game doesn't have AI or any kind of sorting system that does work for you ala Source's new steamworks file compiler.
Say you wanted to put blue thunder's body on a Willman instead of a CRD or patrick it's on now.
Since the SM meshes aren't seperete peices for body/ricerback/bed/hood, becacuse flexbodies are apparently scary alien things from the future's dystopian world ruled by the antichrist and his dog moophy, it's all one mesh.
Go into the truck file (open with notepad) and hit Control F. Now type in props
28,25,9, 0.45, 0.49, 1.05, 0, 90, 0, bluethunderbody.mesh
that's the placement, followed by the mesh name, the game will the look for the mesh in the folder that the .truck nests in (along with everything else on the truck) Why we don't have seperate folders for textures/models/materials/other guff is beyond me. apparently that would just make too much bloody sense.
anyway, that number is as follows
;ref,x,y,offsetx,offsety,offsetz,rotx,roty,rotz,mesh
Ref means which node it uses as reference, so where the model's origin will sit if all the numbers are zero. Offset X/Y/Z is how far and in what direction you want to throw the mesh, while still using the original node as a parent for rotation/location in context of the truck itself. X is left and right, Y is forward and back, Z is up and down.
Rotation XYZ is the same, just rotation. X rotates from the "Front" of the model origin (But this could be from the "top" if you modeled like a ponce), Y rotates from the "Top" (or again, from the "Front" if you're a ponce), and z rotates from the "side". unless you're a ponce, in which case good bloody luck.
anyway, copypaste another truck file on a wilman (or the willman prop itself) and copypaste that mesh placement into the final .truck you want.
there's more to explain, but frankly i'm tired and i'm going to model things while furiously humming the animaniacs theme