Transmission Lifting

[tf95ZR1]

A few weeks ago I posted how under hard acceleration my gear shift lifts as much as 1.5". The motor isn't hitting the hood, so I didn't think it was the motor mounts. I put the car on jacks and tried to tighten the (2) front and (2) rear C-beam bolts. They seemed tight. I have had the C4 Beam Plates by Bill Boudreau on for about 3 years, no probs- I think they are great.
I gave up and took the car to my Chev dealer who I "kinda" trust. At first they said everything looked good: bolts were tight, motor mounts looked good, and to not worry about it. I didn't settle for this and after seeing the rear of the engine and trans lift under load, they put it up on a lift. Although they were tight, they still thought the movement was at the c beam bolts, and they tightened the bejesus out of them. We're talking basically breaker bar tight.
PROBLEM SOLVED, NO LIFT OR MOVEMENT now.
(Finally) the question- I remember reading that overtorquing can damage the rear of the trans. Yet this fixed the problem. Any suggestions? TIA

Ted