Coolant loss while changing block heater

[428 INTERCEPTOR]

Does anyone know how much coolant an 02’ 7.3 power stroke needs to drain out in order to change a block heater? Another question; what’s the best sealant, if any, to use on the new one? The reason for asking is that the factory one only has two prongs and uses a spring clip to ground the outer casing. The new replacement has three prongs and is internally grounded so the main body does not necessarily need a good metal to meal contact. To confuse matters further, the new main body is brass. It should be self sealing but I have heard of a lot of people with leaking block heaters. Now that we have snow on the ground here in Colorado this is a job I only want to do once.

Thanks,
JJ