Cat C9 engine: Thermostat(s) replacement

I have always called Caterpillar with my engine serial number, and they have been very responsive. I too needed to replace two thermostats in our C7, called Cat got part numbers for thermostats and gasket. Our local truck repair center had them in stock. I installed, but have a leak. Called Cat to discuss installation procedure. Cat said I did it correctly. I ordered two more gaskets that will be in tomorrow. Possibly I didn't get all the old gasket off the block and top cap. Bought a scraper from local auto supply, and I will try again tomorrow. One suggestion I received was to drain about 2 gallons of coolant before beginning and then just before installing the thermostats add the coolant until you can see the level where the thermostats are installed. This will prevent an air bubble that can create temperature sensing problems and proper cooling.