How to proberly Cut a Chassie to shorten/Lengthen

[GmtGmt]

I have gotten a new idea for my Heavy Hauler Project. Im going to make it a car hauler just like fine69s truck. I know these where customs made trucks back in the day, going to make my own bed so i get how i want it.
thinking of extend the chassie behind the cab like 2 foot so i have a 10 foots bed.

the only real problem is its illeagle to do this kind of mod to the chassie really but what they dont know they dont complain about

so i would like to see pics of how its done proberly and it would have the same strength as the original chassie has

we are talking about a 1986 C30 Crew Cab dually
its an old Western hauler truck

going to remove the rear seating and put a wall behind the front seats and make the rear a complete work shop with every kind of Auto tool you could need at a race track or side of the road.

This is what i have in mind just a quick rendering not done yet