B Body Mopar chassis stiffening kit, $650. Or get the next level of full race stuff, with mini tubs, rear spring relocation for 400 bucks more

this is the $650 kit

A total unibody stiffening package for your 1966-70 Dodge / Plymouth B Body. Includes cnc machine cut Frame Connectors, a complete set of front & rear Torque Boxes, Inner fender Braces and a Lower Core support frame brace. This is a complete chassis stiffening package that will add an incredible amount of rigidity and structure to your B-Body unibody.

this is the $1050 kit, the same as the top, but with the mini tubs and rear spring relocation added

http://store.uscartool.com/Chassis-Stiffening-Kits_c_46.html