I only have a suggestion for your very last sentence. (Yeah, so trivial isn't it?)
Use your alternator's charge light to turn on a relay to connect your secondary battery(s) whilst charging. Being '85 vintage the alternator (Nippon Denso?) L-circuit should have no problems powering a typical relay.
And re fusing, I'd suggest self resetting circuit breakers at each end (up to 50A) and a relay to suit - ie, 60A, or 30A if fusing is 30A or less.
Search for uibi, or google 'uibi oldspark' for more info. Or maybe better still, just ask.