I assume something of that era had a regular (ladder-type) frame. I would start with the frame, probably from strip stock basswood, maybe styrene. Not being familiar with the type, I am not sure how best to do the body. Does it have a lot of compound curves? If so, carving double-curved parts from wood would probably be best. For flat panels or single curvature, styrene would work fine.
If you can find the scale drawings, fine. If not and you have access to one that you can photograph, here are some tips on photographing to make scale drawings from photos.
Get as far away from subject as possible and use telephoto. This will reduce perspective errors.
Photograph 4 views (front, rear and both sides) from the middle of each side (end), with camera axis perpendicular to the side/end. This is very important to reduce perspective errors.
Place a yardstick near or on truck to help with scaling. Measure one or two important dimensions, like tire OD.