Jesus battery cable, that is so seriously not needed, a 90 amp alt will put out continuously around 40 amps if it's a good one, the 90 is a peak for a split second.
Some much thinner wire such as 56/030 is more than up to it, 76p a metre from
vehiclewiringproducts.co.uk this is thinwall which will be much better with the heat of under bonnets also, this carries 39 amps continuously and is similar to what I've used, I used 2X 28/030.
If you use gold plated terminals on the wire this would be even better.
tabetha