Rob;129092 wroteMore negative caster will help a little if you can get any out of it.
Negative camber, or castor?
I used to have about 4 degrees positive castor with 2 degrees negative camber on the old 144 and it made the steering a lot lighter - and that was with manual steering and 205 tyres on widened steel rims. Going from the factory castor setting to +4 made a huge difference to the feel of the steering, after I'd initially had it set for -2 deg camber, but factory castor setting (0 to +1 deg).
Standard settings on the 164 are 0 to 0.5 deg of positive camber. With the castor, they changed it from 0 to +1 deg up to '72, then +1 to +2 for 1973, and then went to +1.5 to +2.5 for 1974-5.
Perhaps a wheel alignment with some less conservative settings than factory, along with a power steering pump fluid flush, may improve things.