Rob love the solution for the rear support below the ZF...I did something very similar, but actually laid a 3/8" section of plate all the way across and it actually crossed over the frame section running the length of the car. Your idea of the half round tube welded to the underside of the plate is pretty sneaky to strengthen the plate!!!
I noticed there are heat marks around the sway bar mounting studs. Did you do anything to those? I cut the bottom of the mount channel off and slid a 1/4" piece of plating up inside that frame area and it is threaded where the two old bolts were...that way you can thread in new higher quality studs for the sway bars.
Also for the emergency brake, you can cut the old one off and there is a bolt on bracket that I purchased from one of the vendors that I'm sure you can make with your skills. It spans between the two frame rails and fits flush to the frame with the brake cables running on top. I'll try and find you a picture and post it.
While you are at the rear sway bar mounts on the rear A-arms tend to flex and bend...so I added a piece of 1/4" steel on the underside of the bracket and it is also threaded so you still thread the bolts into the existing nuts, but the holes in the 1/4' steel are bigger so the bolt threads through. Might be overkill but it's strong. I attached a picture.
You are making some incredible progress!!! Keep posting the pictures.