do you have a multimeter if so just put it on the battery with the engine off it should be about 12.3volts with the engine running from cold it should be 14.2 volt +- .2v
if you get 14v then it may just be a lose / bad connection on the charge light terminal which controls the light on the dash.
see image on page 3 --->> http://www.autoshop101.com/forms/alt_bwoh.pdf
yeah the 120a is the biggest you can get unless you send to off to be rewound (£200 ish) then it can be any thing from 150a to 170a
you should not need any more than a 100-120a unless you have a silly stereo setup like mine but that was fine with my 100a and 2 batteries 70a at front and 97a in the boot on a duel-charge relay
i have a 100a alt in my workshop unused .. 🙂