Should be no problem with the heater core. Problem is the heater valve.
Sorry to say Fusick uses rubber for the diaphragm. I talked to Terry in the back who does their valve rebuilds. Everyone i checked with uses rubber. According to Terry and the Evans guy Dexcool and other modern antifreezes with Similar corrosion Resistance chemicals also have a problem with rubber. Modern hoses are not rubber, they are modern compounds so you don’t see a problem there.
The good news is in talking this out with Evans and then Terry, Terry agreed that a modern “rubber” like Viton is resistant and in checking it out Terry said they will work up a Viton diaphragm. It will cost slightly more for a rebuild with the Viton diaphragm, maybe ~$20. Hopes to offer it beginning of 2021.
Since i already have Evans in the system I will pop for one. My valve has the capillary tube clipped so replacement is simple. On the other hand if your engine is installed and the valve has the tube it’s a lot of work to change it with the engine in the car.
The Evans works very well. I run a 180 Degree high flow Robert Shaw type thermostat from Stewart Eng with a small bleed hole in it. Usually run a temp below thermostat Rating. Don’t know if it was a fault of the Evans, which is less viscous than standard antifreeze, but i had a hell of a time clearing what i believe was an air pocket on my system. Had a running hot problem for a while On the new engine which suddenly solved itself. Had some of what i believe were contributing issues which i wont go into here.
Like the corrosion protection and not having to change it every 2 years. If you loose some you need to re-top with Evans or you loose the benefit. Its pricey, but you don’t have to change it.
I run Classic Green in my other cars mainly because they already had a std antifreeze in them and
To change you need to flush with an Evans prep product and completely eliminate any water and then fill with Evans.