Being as far south as I am, I don't have the problem of winterizing that you will. But so long as the water does not completely freeze down here, the fish normally survive. If your tank is deep enough so as to not freeze below the freeze line, they should be ok.

Your 78 gallon tank is indoors, right? They'll be fine there, for now, provided you have enough filtration, etc, but will continue to grow all winter if kept indoors in a warm tank. You might not have enough room for them next year, depending on how large they grow before then. Koi do need more water than that but if you have a lot more filtration than you need, you can get away with less water per fish, if you're careful. Test the water often, make small water changes frequently, salt in the water, don't overfeed, etc. etc.

My 3 good-sized goldfish (about 8") spend the winter in a 50 gallon aquarium in the garage (didn't want it in the house). I simply remove enough pond water to fill it about 70%, add some aged clean water, use their pond bio-filter/pump and that's it. They spend about 5 months here every year (for the last 10 years, same fish). I do a partial water change and clean the filter about 1x month and feed them lightly maybe every other day while in the aquarium. It's worked well so far!