I wouldn't assume that the original alternator is on there - nor that a replacement is exactly the same. You've got 35 years of who-knows-what's-been-done-to-the-boat going on. It could be the same - but assuming you're asking because of fuse sizing - probably best to find out for sure.
The device in your link, is an ACR/VSR. As stated, with the simple 2 post ACR's, the wiring is such that the main cranking always gets alternator charge. The device has zero bearing on that, despite the claim in the link.
States its waterproof, but is it ignition protected?
Yes, dropping anchor and leaving a 1/2/BOTH switch on BOTH, could leave you with 2 dead banks. However, this chance is slim, and far out ways the cons of eliminating the dual bank switch all together.
Not a fan of having large loads wired direct to a battery, with no main disconnect. Your plan is to just that. here is a far better switch design to pair with an ACR, a Dual Circuit Plus switch. Gives you a main disconnect for when the boat is not in use, yet divides the banks in a dedicated main and house.
