Speaking (or writing) from experience, the sheet steel of a uni-frame and/or body parts, can be made some softer than it is (after all it´s sheet Steel). The weld being the hardest part of the patch, from added carbon and the temp. radiating from the weld (uncontrolled temper). Often end up, with very hard, to some hard, to softer, then back to original tension strength. It´s not a very controlled process, sometimes the weld or very near the weld, cracks, from disimiliar hardness, sometimes next/near to the weld area it can crack/tear from softness. Temper is a pretty exacting temp. controlled process, with welding you have many temp.s radiating from the weld out.
But heck the patch is often stronger than the rust, so it´s an improvement anyway. Heat can be controlled somewhat (localized) by spraying the surrounding area with a film of water (from an old window pump sprayer), doesn´t bother a wire welder a bit and makes the demarkation lines of different temper/temperature, much narrower, (the areas that flex, expand and contract at different rates).
Much of the problem isn´t the temper, so much as dissimiliar thicknesses and strengths, next to each other. A larger patch spreads out the stress over a larger area. A 2MM thick sheet steel, is easy to weld and much stronger than the original frame, good material for patches. 1,5 MM is acceptable on most areas, have to study/estimate the stress. I usually, think of the process as controlled crush, I´d rather have it bend than break.
Making the side frame rail patches some longer than the bottom/top frame rail patches, helps distribute the stress and adds a lot of overall strength.
Using a very much thicker metal for the patch than original, doesn´t help much, just, moves the stress points. Continious welds are seldom necessary, spot welds and/or short runs, help with heat control.
Have heard, the newer XJ´s use a water hydrolic press process, that realigns the metal (chrystal) structure and improves tension properties (makes it harder/springier).
I´ve been welding awhile also, like to watch the old guys, always something to pick up.
Thanx for the site, Colorado_XJ. I weld by instinct, don´t think about it much anymore. My sons in school for his certification.