Been pondering your floating sub-frame problem and have been chewing it over with Paul. While most 70 Fury models also use the floating frame, convertibles retained the older fixed style to increase overall car rigidity to compensate for the lack of a roof. I find the convertible ride to be very solid. As you've discovered, '71 cars all got floating sub-frames in an effort to isolate road noise. The also had isolation pads to "float" the rear springs from the rear end. One side effect, as the rubber got old, everything loosened introducing squirm. A comment Paul made, if the front clip can move about on it's rubber supports, what does that do to panel alignment - specifically between the rear of the front fender and the door. Paul is very anal when it comes to panel alignment such that all gaps must be narrow and dead-eye straight. Not sure how to do that with everything moving around. Gina's son can spin up some aluminum spacers on his lathe which would convert your floating sub-frame to a fixed one like mine. I recommend you try this and if you don't like the ride, the spacers are pretty easy to replace. Aluminum should last forever, won't rust and as long as the bolts are tight, won't wear. Your thoughts?
Forgot to add, go all rubber or all aluminum, don't mix them. I noticed in a tech note that the floating frame bolts must be tightened carefully to avoid ripples in the sheet metal. That along scares me off.