I've borrowed a number of parts from P30 chassis, or if not P30, then similar GM chassis. (wouldn't call them a big chassis though)

Whatever they call it, you most likely want the one with 10 lug rear and 5 lug front wheels, like your truck has now right?
I prefer the ones with the straight I-beam front axle, not the independent front suspension.
Either way, with that bolt pattern they will have disc brakes front and rear.

The chassis is very straight and simple, resembles your original, but I don't know how close the actual measurements come. Check the track width to see how well the tires will fit under the fenders.
Best bet is to take a tape measure with you and check out the local delivery trucks with that axle combination, if measurements look good then start looking to buy one.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-