The original wiring to the dash from the battery is 6mm2 you might be running to much current through it now. Before you play parts darts measure the voltage at the battery with a multimeter then at the fuse box to check if there is a large voltage drop. Check all the connections on the original wiring are clean. You may need to consider running an additional cable from the battery to the front of the vehicle to spread the load.
I have a 70amp alternator for sale: http://thelatebay.com/index.php?threads/lots-of-bits-alternators-heads-barrels-seals-and-more.79208/ Believe i have the cooling boot for it too that goes in to the fan housing.
just a quick update..tried the new alternator and it fouls the tinware at the back by 5mm, so that was a no go. changed the regulator and replaced the alternator with 1 I had kicking around and now I am reading 18v on the digital readout but I have to yet buy myself a multimeter to test it thoroughly, but 18v seems a little excessive and worrying
18V does sound scary high -- your battery won't last long - but check the voltage directly on the battery. Measuring up front doesn't really mean anything and is just misleading.
Your battery does not have a good connection to the alternator. Or its totalled because of all the time it has not been charged properly.
