[johnsjmc]

The pressure recommended was something like 18 f 28 r or even more. In the 60s gas stations still had attendants who no doubt "fixed" the soft fronts and set the pressure at 25 all round . I think this more likely than underinflated rears
The low fr pressure induced some understeer. The Spyders had front sway bars from 1962 and they were added across the line from 64 up.
911 in the 60s had equal sized tires and the 911 " fixed "some of it,s driving charactor with staggered tire sizes and soft rear torsion bars.
The Corvairs were also sold to little old lady drivers who would already be spun around before even knowing the car was sliding and 5 turns lock to lock manual steering also didn,t help you catch anything during a spin.