Subframe connectors for a driver/cruiser, necessary to weld to floor?

I'm getting close to installing USTC subframe connectors on my '65 Barracuda FS project. The reason for installing them is to (hopefully) eliminate the body flex that causes cracking at the base of the rear window. My body man did put extra welds in the corner joints, but also encouraged me to install subframe connectors. I've got a lot of $$$ in this car and don't want to risk the return of the cracks.

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