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So my first question is do you think that the cab is roughly the same dimension in the rear as a 1/2-1 ton trucks?
Yes the cab is about the same, which is to say that the firewall & Floor are were the differences are. The rest is the same. Same doors, same windows, same dash.

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The second question is what is your opinion of the big V6 engines for use in a tow rig?
The GMC V6 was built to pull, they developed lots of torque for pulling big loads. I'm building a 1965 GMC 1 ton with the GMC 401M V6 engine, New Process 540-GL 5-speed, & Spicer 5831-B Auxiliary 3-speed Transmission. Should be a pulling brute.

Now would be the point I would tell you to check out the, 6066 GMC Trucks website, but looks like you have already.