I've had a lot of experience with this sort of work too. A lifetime's worth to be exact. Having done, engineered, supervised and instructed others on hundreds of frame modifications I would NEVER cut a frame " directly behind the cab". Most all frame failures occur within 18" of the back of the cab so it is best to avoid cutting that area and place the cut as far behind the cab as practical.
Things to consider when doing any frame modification;
A. what are you wanting to do with the "new" frame? Simply increasing/shortening the wheelbase or adding equipment?
B. what new equipment will be added and what are it's capacity and limitations?
C. what is the GVWR of the unit as manufactured?
D. what is the new higher GVWR desired?
E. how will the truck be used?
F. special considerations involved such as off road use, cranes, winches, etc.
G. frame materials & application
With that information one may make a solid and serious evaluation of the requirements and methods. The engineering work involves meeting the RBM of the frame as designed or increasing it and knowing the section modulus of the material.
Here is where I agree with previous comments: Always install reinforcements with a 45 degree angle cut on the end. I also agree that frames can be made too rigid and need to flex. Having a mechanical and engineering background is alway helpful and especially so here since so many variables are present. Good math skills are a must as you need to calculate the RBM.
Here is where I disagree with previous comments: bolting to the flange. ALWAYS bolt to the web of the material, NEVER bolt or weld on the flange of a reinforcement area. It is quite doubtful that you will be using original frame material which may have a much higher tensile strength than the replacement material so caution must be used not to weaken it with welding or drilling. Frame flanges should never be welded anyway. Always keep the two materials tight against each other. There is no real rust concern if properly cleaned and painted. Most new truck frames never see much paint anyway. Do NOT weld the two materials together, only bolted.
The reinforcement area should never be less than 18" either side of the cut if at all possible. Reinforcement can be with angle or a full channel on the inside or the outside depending upon the design criteria. For an older truck as you describe you may be able to use an angle on the inside with the flange on top and the then bolt it to the web section with a minimum three bolt pattern at the ends. Center bolts are not needed unless the length exceeds 24". 1/4" mild steel plate (36000 lb tensile strength) can be used for the like of an old farm truck, etc. Weld should be done uphand, DC and with a rod material of suitable strength. We normally used 70 series or stronger. An old chassis may only need 50 series and MIG can be used. Leaving a gap between the materials allows for the flexing in a direction you do not want, i.e., outward. Al welding needs to be done by an experienced hand with good penetration and appearance. Grind the material to a Vee and use fillet welds with appropriate rod size so it is done in one pass on these light materials.
The reason I go into such detail is obvious to most but never to all. This is the backbone of the truck. If it fails it will have dire consequences that can involve the operators life or worse, others lives. I do not intend to scare anyone off from doing this work but like so many things it is necessary that you know what you are doing.
Ruining a frame is probably going to create an irreplaceable situation. That and the fact that none of us wants to see an old truck screwed up, only rescued and driven.
