YEah cutting and shutting (as its called) is always a dificult choice where to cut
i think your original idea was probably the way to go, keep the entire cab from your new 82 vehicle, or was it not a chassis cab but a minibus ?
if it is a chassis cab, then i would start by cutting the new one off behind the cab, then proceed to remove about 12 inches of chassis from underneath (but not going as far as the back front spring hanger, don't want that headache) then take your recovery and remove the cab expossing the chassis just under cab at the back, thus allowing you to cut the chassis off at the same point as the other.
Then to rejoin i would sleeve the inside of the chassis, but before welding it all together, i would weld a solid metal bar onto the underside of the original chassis and leave it extending past the joint, this will give you a very handy guide and strengthener later.
Once you have rejoined it and welded it all up, i would then look to a mk4 transit (tipper or flat bed) and look at that same area, and you will see that the chassis is "double" height, and higher than yours, so much so that it is visible from inside the cab, i would take that part from a mk4 and add it onto yours (about 24 inches or so) This way you will have braced the joint top and bottom and it should never give you any problems
the only thing i would be concerned about is the number of pieces of metal together in the one place, as this usually encourages rust, but a good coat of waxoyl or expanding foam should take care of this.
You could also, take this opportunity to either extend the chassis and flat bed or extend the chassis and cab
