70A is standard 3DR and indeed for alot of models, yes for the pulley on the front of the alternator you may need an "impact wrench" as it may try and turn the pulley otherwise when you try to undo it.

Rewiring makes no difference, there is no extra load, it just means the pump is getting the power it should do and so you are not left in danger due to the old wiring, whereas on the now old wiring it may not be able to as the wiring has builds resistance.

The fan loom is only half the problem, ideally you would have a switch with a better span fitted, known as an "Uprated" switch.

There is no reason why the 70A should not be upto the job, no all 90A would mean is the alternator can give a higher output when under needed, but if you don't have that demand then there is no point.

This is what later high spec models and Granadas had.

You should get a regulator otherwise I would be walking away!

Martin