Sounds like overkill to me but if it makes you feel better,go for it. I would attribute the flex and twisting to a inferior
suspension. Sway bars,shocks or any loose parts.
If you're referring to my flexing, the amount of frame flex is inherent in the structural rigidity of the frame design and is independent of the suspension design or condition. Further, it's not overkill if the OP is looking for improved ride. One of the biggest contributors to current vehicles' ride quality compared to vintage ones' is chassis rigidity. Springs and shocks, while certainly improved, are not multiples better whereas torsional rigidity is. And if he's dropping in even a stock 350 the frame will be subject to at least 50% more torque than the original 216 for which the frame was intended. Now, doing MPI on the frame MAY be overkill, but hey, if it's free, why not? It's a whole lot easier to repair the weld now than after the first coat of wax and a test drive.
