It’s a good idea to do a leak down test on the water jacket after you rebuild, before putting it back in service, filling with coolant, etc. If everything is serviceable, you use good gaskets and assemble properly, it should pass this test. Based on your description, I would think your engine would not pass this test. As to where it is leaking? This test does not tell you that! Depending on your cooling system design, overflow tank setup, etc. it is possible to lose coolant other ways. This test rules out the engine water jacket as the escape route.
See attached shot of the leak down tester. It has three holes that match the bolt pattern of the cooling in/out passages on the front housing. The water pump housing (if any) and long studs if installed, need to be removed. Then you bolt this to the engine block covering the in/out passages so the gasket makes a seal (made of white rubber sheet in my case, but you can use a real gasket here). Adjust your compressor regulator down to 15 psi and plug into the air tool fitting to pressurize the cooling jacket on the motor. DO NOT EXCEED 15 psi or you might damage your seals by testing. Shut off the valve and disconnect the compressor hose. Give it overnight and see if it loses any pressure. The drop should be nearly undetectable - 1 or maybe 2 psi can be assumed to be coming from a leaky valve or poor gasket seal between the checker and the engine block. If you make one of these, use a high quality valve. The depicted one was leaky and I had to add a blocked off quick disconnect that I plugged onto the end after pressurization.