Before looking at subframe connectors, look into solid body mounts. Get your frame locked to the body. This stops most body-to-frame flex by unifying the chassis and frame.
Your best bolt-on connector will be from Pro-Touring F-Body. The PTFB (Pro-Touring F-Body) subframe connector actually ties the frame sections together which is the structure of the car.
http://pro-touringf-body.com/chassis_components.html
Your best frame connector for chassis rigidity is probably Detroit Speed but the issue with their setup is the intense amount of work. You need to cut through the floor with their setup and weld everything. Far from a bolt-on beside...
https://www.detroitspeed.com/1970-19...irebird-010103
I would stay away from the Competion Engineering subframe connector. That setup bolts up the the spring pocket and clamps on top of the frame bushing. Not a very strong design.