T-bars vs Stored energy

A few things come to mind. First video looks like the pinion angle is positive allowing the rear to pogo when it meets resistance. Another note is that a stock pinion snubber will have flex and a soft pad whether that's rubber or poly. The bolt mounted to tubing welded in the car is too stiff. The problem is that your getting too far away from Ma Mopars million dollar engineering and not buying a complete system for the rear like a Calvert set up.

The second video shows a car that's too stiff. It doesn't allow the rear spring to separate driving the tire into the ground. My stock suspension cars worked best with 6 cylinder leaf springs with a good shock. Most of your hook is in the front suspension. Especially in a small tire car.

In conclusion I would use a blanket approach. Easiest way is caltrac with ladder bar coil over being a close second. Then concentrate on the front. I spent a year trying to make super stock springs work on a 10 second car. What a waste of time.