Will be watching this build
I assume you just don't want the exhaust to go over the stock cross member?
In your design, you could eliminate a few welds if you just bolted it to the top of the frame rail C vs the bottom which would help with strength.
Analogy: You want to create a good 3 legged stool for the engine and transmission to sit on. 2 motor mounts and 1 transmission mount make up the legs of the stool. This way as the frame flexes your motor and trans are supported evenly. If your transmission cross member is flexing in addition to the rubber/poly mount, the engine motor mounts will suffer as they will be trying resist the extra flex.
Look for something you like and copy it. You don't have to use pipe/tube, but suspects it's 3/16" thick so it won't flex as much.
