A Pair Of 894-895 - 1.03 Torsion Bars

Kindof depends on your definition of mushiness.
I would start by inspecting the Unibody. and especially the aprons to frame and firewall welds. and don't forget the rockers. You may need some frame connectors.
And don't overlook the fact that she's a convertible, with a 55 year old body. the frame sections could be paper thin in key places. and rotten floorpans are not your friends.

Some to a lot of the sway, could be coming from your rear suspension. Such as; under-inflated tires, worn out bushings, over-length un-braced shackles, worn out springs, broken hangers, I mean there's a lot going on in the back. If it's moving around in the turns, it will tend to self-steer the car, which you might, I suppose, interpret as "mushiness."
Empty the trunk too. Getting the spare tire outta there should be job-1........ IMO.