You have two goals that would seem to interfere with oneanother. One is cooling. You are going to have to get rid of about 5 kw of jacket heat either with an automotive type "radiator" or through the surface radiation of a tank, or both. The other is storing heat to use when you want it. This requires preventing heat loss.
In order to use the coolant liquid as hot water you are limited to water as coolant - obviously. This presents corrosion difficulties.
A shell and tube heat exchanger running corrosion inhibited liquid in a closed cycle through the engine is logical and good practice. Plumbing that shell and tube to heat water in an insulated tank is also logical. Plumbing either a tank-cooler or a "radiator" is also necessary in order to prevent over-temp situations.
A normal hot water tank with the insulation stripped off , eg 35 to 40 gallons or so, is too small for a 6-1 at full power and sustained ops. Way too small. With the insulation it is, obviously, far worse.
The heat calculations are well done on the site - eg the size necessary for radiator, tank, and so forth. Look through my past posts and you'll find 'em. I made mistakes and corrections until it got very clear.