Steering ratio changes how light the steering feels. If you have the short steeering knuckles (PS or GTV) then you effectively have a quicker ratio, which will feel heavy. The Nissan knuckles probably have a different length and feel.

As said, caster is a huge factor. With lots of caster, you actually lift the front of the car up while turning. Do it in the car park, see the bonnet get higher. The weight of the engine makes the steering heavy when you have lots of caster.

I recomend zero toe-in, and not so extream caster and camber settings for a starting point. Especialy if you are having problems.