Re: "new" 1975 johnson 6hp
Ah, nice. Collector value? Probably not much. They made so darned many of those 6hp outboards, and most of them are still hanging on the back of a boat. Typically, you can buy a good-running one for $250-$350. Since yours has never been run, you might be able to tack another $100 or so onto it.
If it's still in the original shipping carton, with the tank and everything, add another $100 or so.
Odds are that a pair of spark plugs go in, and it starts up on the third or fourth pull of the rope. I'd change the impeller, just because of age. Then, I'd hang it on the back of a 12' aluminum boat and go fishing, figuring I'd have a good outboard for many years.
One or two unused one of these shows up every year on the market, found in some old dealer's storage room or in someone's garage or basement...still in the original box. They make a heckuva nice outboard, but don't have much collector value at all.