As someone else has pointed out crosswind instability is largely down to steering. Check the steering box. DON'T overadjust it, it will wreck it in short order. The steering box has a high spot in the centre of the worm, it must only be adjusted in the straight ahead position, never try to adjust out play at the extremes, it's supposed to be there.

Similarly check the steering idler for play

I don't really think the IPD roll bar will make a lot of difference. It's a bit like very large tyres. It makes the car feel better, but compare a good driver in a car with a standard roll bar and one with the bigger bars or oversize tyres, the standard one will be faster, but more untidy.

I reckon the standard front axle is preferable to the 120, 140 conversion, simply because it is more adjustable. Again as someone else has pointed out it needs to be in good condition, there is an awful lot of metal to metal contact in there.

You need to establish a baseline for the adjustments. For instance after we put a norrowed 240 axle on the '58 PV we measured the suspension settings to be:-

Rear axle Camber Left -0 degrees16 seconds, Right -0 degrees 27 Seconds.
Rear axle Toe Left +0.6mm, Right -1.3mm Total toe -0.7mm

Front axle Caster -0 degrees 37 seconds, Right -0 degrees 48 Seconds

Front King pin Inclination Left +5 Degrees 56 seconds, Right +6 degrees 08
seconds

Front Camber Left -1 degree 09 seconds, right -1 degree 07 seconds

Front Toe Left 3.9 mm, Right -2.7 mm, Total toe + 1.3 mm

I'm not saying the above figures are right, or even that you should aim to achieve them, but settings are adjustable on a PV and if you don't actually know where you start you are groping about in the dark. Incidentally the '58 PV drives very well.

Certainly if you fitted another rear axle you need to know where the wheels are actualy pointing before making further changes.

Finally remember that a steering box will never feel as positive as modern rack and pinion, the solution is to leave he modern in the garage more often and acclimatise!

Regards

Pete