One method I use for getting all the rough dimensions is to find a plastic model of the car/year you are building and then simply apply the scale to the measurements you take off the model. Not perfect, but it will get you close. For $8-$10 bucks you can get virtually any dimension you need for any body part. I'm not sure I've ever seen a model of the '31 pickup but you should be able to lay your hands on a '30 or '32.
Combined with MK's frame dimensions (above) you should be in business.