The links Stu posted are a great source of knowledge. Following them will necessitate spending extra money
in the short term, but the payoff is worth it. The regulator is almost required and the changing in how the switch is wired will keep you from frying your new purchase
I'd guess you'd be better off with replacing/upgrading the wire. Maine Sail (who wrote/contributed much of the info in the links above) has said that the starter and charging
circuits tended to be undersized 20+ years ago. You don't say what boat you have, but if it is from the 80's, you're likely undersized from what a manufacture would install today (being 0/2).
Assuming you don't have starting problems now, the real benefits of a large wire would be to allow that new, fancy alt to output as much as it can / wants to.
Plus, if you following the advice in the links above, you'll be replacing all the wiring, so why not do a quality job?