Barracuda sag

I presume that the subframe connectors are welded in at front and back only? This is the advantage of the US Car Tool connectors that weld to the floor pan. You might be able to stiffen things up by welding in some plates that attach the connectors to the floor pan. Not the whole length, but I would put pieces in on both sides of the subframe connectors where you do, so you make a little box tube between the connector, the installed plates and the floor pan.