Pauly
Printrade.com.au
I hope whoever originally wrote it chimes in, but someone once mentioned taking the picture from as far back as possible to prevent lens distortion. Something about keeping the object in the middle 1/3rd or 2/3rd of the objective. Also if you have a cab that is offset from the box, then sizing to the box will make the cab small and sizing to the cab will make the box too big. So for proofing you'll have to estimate that, but the production file should be just the box and just the cab.
You photoshop RAW editor to adjust for lens distortion. Basically find your camera and lens in the drop down and it corrects it, including the vignetting that you wont notice until it's gone.