These older cars are so flexible that almost any added structure is a noticeable improvement in rigidity. A case in point: I had a '73 Nova hatchback and a '73 Z28 - likely less rigid than a Mopar due to the GM separate rubber-mounted ft subframe - and I put a pair of those bolt-on Competition Engineering connectors on each car. The improvement was VERY noticeable; the Z28 cornered more predictably (it was pretty much a SoCal canyon-runner) and the Nova quit creaking and was MUCH more solid. I'm sure both would have been even better properly welded & tied into the floor, but the point is even the simplest stiffening was a big improvement. FWIW I know of another car that has welded-in round tubing connectors and the round tubing still allows a little flex; IMO, square or rectangular tubing works better.