Great to see someone putting the time and effort into an old rig. Just because it's old doesn't mean it's anything else, or has to be.
That switch panel looks great. I would build an engine cage to tie into your shock hoops as this slows them to brace each other but also adds rigidity to the frame via triangulation.
With these C-channel frames the last thing that you want to do is create rigid section in it. Where the rigid to flexible transition happens will always crack. Some of the CK5 folks box their frames, but that presents a whole new set of problems because everything about the frame was designed with that flex in mind. You have to re-design how each cross-member is built and attached or they crack where they're joined to the now more rigid frame rail.
If an engine cage is built it should tie to the frame with urethane bushings. That relieves some of the stress from frame flex. The best way to stiffen the frame is a well designed and built cage that attaches to the frame, but such a cage will get in the way of normal use of the vehicle.
I am concerned a bit about the doubler cross-member. The original powertrain mounting was 3 points for a good reason. With three points bending and torsion stresses on the power-train assembly from frame flex are almost impossible to have. With 4 points three will carry the weight and the 4th will add twisting or bending stress to the power-train. The usual place that this shows up in GM auto-trans' is that the output flange on the trans case cracks loose from the rest of the case. If this happens consider going to a single engine mount under the crank pulley.
