With boating many times "because I want to" is a valid reason. This is especially true with the guys who want to convert to diesel and they look and look (and look some more) for reasons to justify the conversion from gas to diesel, and most of the time they don't even get close. Therefore we just say, if you WANT diesel and you can afford it, hey what the heck diesels are great if you have em, but they cost a ton to install and they are not going to take care of themselves once you do install them.
The closed cooling does have the benefit of being a winterization for the parts of the system that get the antifreeze, but the parts that don't like the sea water pump and the oil cooler on the oil pan, and the transmission oil cooler, and the outer jacket of the heat exchanger itself, do require winterization. Lately I have developed a system for runabouts where I run them until hot and then circulate a known strength of antifreeze through the motor until I am sure I am getting eveything around the t-stat, etc., and this gets me through the winter just fine, the motor is well protected internally and I can fire up and go anytime I want. Naturally if you do this you MUST use the environmentally safe antifreeze especially if you have dogs in the family like we do.
Having the boat equipped for salt water could be fun. This hull was a real hit in the Mediterranean when Herb Pocklington built a variant with 427 V-drive power called the MONTE CARLO. The offshore racing technology Jim Wynne designed into this hull would make it quite comfortable out on big water.
The fact that the MM expressed interest is noteworthy but no reason to not equip the boat for salt water or for upgraded protection, because if you do the serial numbers STILL all match just like they did before, and any buyer would always have the rather easy task of removing the closed cooling and converting back (especially if you save the spedific parts in a kit to allow any future buyer to do this). On a boat like this it would be nice for any future buyer to be able to get the conversion pieces with the boat, whether it was standard or closed, it would be a nice option at the time of the sale.
If you are getting ready to do a restoration you sure have some time to make a decision like this, but if you see a complete system and you know you are going that way, my advice is to get it when you can because if you decide you want to do it on the short term it could well be months before you ever see a system for sale. If mine are no longer available due to sale, I will help you find exactly what you need because I have a complete inventory and photo documentation of the required parts, even the small copper tube that comes off the bottom of the surge tank to handle air pockets
One thing that is a bit off topic with this thread.......but when I looked at the motor I was a little confused about the intake manifold at the time, and it may be because the motor is installed backwards to work with the v-drive. Take a look at that manifold and send me the numbers, I have 3 iron 427 marine manifolds in my shop now, and it will be interesting to compare just exactly what you have on that big dog. This is not to suggest you have the wrong intake manifold, just to say I was wondering about it at the time. Since I made that comment I have become much more familiar with the iron FE intake manifolds, lol. 80-pounds of industrial srength American iron.