The earth one needs to be much bigger, that shown is only 16m^, you really do need as I said 196/040, which is same as jon's 25mm^, carries 170 amps, as opposed to the one in your link carrying 110 amps, of course you could use much thinner for the smaller one to the battery tray area, here 84/030 would be more than ample!!, carrying 50 amps continuously, which you wouldn't use in a million years not continuously.
For the alt, if you want to REPLACE the wire use 84/030, ford did not use the same gauge wire for the whole run to the alternator, around 12"-18" into the loom it gets fed through a connection point, which is a sealed soldered joint, which corrodes terribly, and also goes from two wires into 1.
Although the alternator may be able to put out 90 amps(and most don't put out as advertised) it would die as a continuous load, my dads xr4i for example has a 34 amp alternator and manages fine, it's just nicer ford fitted a big powerful alt so it is not overworked, I'd be amazed if it was using over about 40-45 amps in use though.
Btw, www.vehicleproducts.co.uk is cheaper, and you may find polevolt have a minimum order, and that you need an account, so if you get stuck give me a shout, as I have one.
tabetha