How much chassis stiffening needed?

Floors front and back and trunk went soft and holy in my 340 A/T 69FB. The first apparent observation something terrible was going wrong was when the leaded seam between the sail panals and roof broke at the top corner of the back window on both sides of the car. Good and proper floors are essential to the torsional strength of the unibody. I repaired the damage and replaced the floors and patched associated rusted frame pieces. In anticipation of a 550 hp BB conversion I then installed fully welded USCT connectors and 4 torque boxes, then upgraded to 1.03" bars. What a difference! The car now has the solidity of a brand new modern vehicle. After pounding on it hard for 2 years there is no sign of damage to any welded seams or the paint finish due to body flexing.
A friend installed the cheap connectors that only bolt and weld into the front and rear frame horns on his 70 CUDA 416,, after a week of spirited driving he had chipped the paint at the 2 rear corners of his doors where they meet the quarter panels. The floor cannot be too strong.