Jim,
First for the threads, here in Holland most guys use threaded rods, you buy them per meter. You can get them in 12.9 quality. Nuts on both sides.
Not cheap but you can do two cars with a meter if you cut carefully. If you want to go cheap you could do only the front ones
The strength difference between course and fine thread is not really big just go for the better quality iow 12.9 and matching nuts.
Then some remarks on your stiffener sketch.
So I see again a U shaped (two folds) in each bracket, so you do not generate any stiffness. Thickness of the U's?
What I'm not sure of... are you going to weld these into place onto the outside of the chassis members?
If so I'm still not convinced it is the best idea, but is already ok-isch.
To get this more to my liking
Would you consider opening up the chassis member and put in a solid square block, instead of the alfa original crush tube? And weld it into the chassis member?
If so... we have to work this out a little better
Then for the bolts to secure the connection strip between the two members...
What thread diameter are you thinking about?
General bolt connections:
For a good bolt connection you need at least 3~4 bolt diameter free length, between head and the first engagement of the thread. Long story short, compensate setting behavior and leaving enough elastic pretension in the bolt. What you are sketching here isn't going to stay in place very long!
I'm not sure if this is understood correct, will make a sketch tomorrow night.
Don't have the right drawing tools / time right now