The idea is not to have a straight cut. It is better to have a miter or a stepped cut. Both enable the top portion to rest on the bottom portion of the join. This of course means that the downward load is not on the weld. The "Z" is the miter cut, you can simply cut the whole frame, including the top and bottom flanges, at an angle. Most do 30 deg...or you can cut the flanges straight and the side of the channel at an angle..... Or you can do a stepped cut, cut down to middle, then horizontal, then down. In the stepped cut you must have no 90 deg corners, you will have to mate radii to radii. Large'ish type radii, depending on height of rail. I would make a good pattern that can be used on both sides. I would not measure a scribe separately. I also would not use a reciprocating saw.
There is a term "Z ing" the frame. This is a totally different thing used for body dropping/lowering.
