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I am offering these comments up on the basis of my experience with a 1971 142 E with separate overflow tank with pressure cap on the overflow tank. I am assuming your B20E has a similar arrangement.
You pressurized the cooling system at 10 psi and tested for 10 minutes. Did you pressurize the overflow tank or use the radiator fill port for the test. If you used the overflow tank for the test, did you watch the fluid level in the tank? If it moves at all during the test, you know that you have a leak somewhere. If you used the overflow tank for the test, I presume that you have otherwise checked the overflow tank cap to confirm that it holds pressure?
If you used the radiator fill port for the pressure test, you need to examine the radiator fill cap (because it is not part of the test). I had a brand new radiator fill cap and discovered that it was not sealing. I had to add a couple of pieces of brass shimstock under the rubber gasket to get it to seal.
You said " When shut off before getting too hot, you can hear water boiling in the engine". The only way that water will boil in the engine (if you haven't greatly exceeded the normal operating temperatures or lost a whole bunch of coolant already) is if the system can't maintain pressure. Normally that would send me back to check the pressure cap or the radiator fill cap (which are the only uncertainties left over from your pressure test).
Are you by off chance running deionized water coolant rather than a conventional water / glycol coolant? If so, the water, wherever it is leaking from could be flash evaporating leaving no residual evidence. Water / glycol does not evaporate quickly and as such will generally leave gooey evidence of the leak location.
If you are running water / glycol and it is leaking into the combustion chamber, this will usually show up as white smoke in the exhaust (not steam) and the exhaust may have a sweet smell to it. If the leak is significant (yours sounds like it is) this should be obvious Check for this on a hot day with the engine up to operating temperature. If you are running deionized water, there may be no evidence out the exhaust pipe that you have a coolant leak. Also, if you have a leak into the combustion chamber you should get combustion gasses in the coolant which you say is not happening.
I rebuilt my B20E completely 3 + years ago including all new radiator and heater hoses. When assembled, I made sure all the hose clamps were tight. After some period of operation I was losing significant amounts of coolant. Inspection showed that the new rubber hoses had relaxed allowing a slow seep from just about every hose clamp. Of course, the worst offenders were the clamps on the back of the heater which allowed coolant to leak down the firewall and underneath the thick underlay under the carpet. The underlay I used was mass loaded vinyl with a thick foam isolator which was very effective at hiding what was going on for a significant period of time. That was one sticky mess to clean up!
After checking the above, as a next step, acquire one of those leak test kits with the UV dye and UV light. They can be very effective at tracking down leaks that are otherwise difficult to detect visually.
Others have raised the issue of air locks and the quantity of the refill. If you are uncertain about this, I suggest that you drain the system and then do a measured refill. If your measurement matches the coolant quantity in the service manual, you can be pretty confident that you do not have a significant airlock. If your refill comes up short, then you have an issue to address. A large quantity of air in the system will prevent proper pressurization of the coolant with the potential for localized boiling (explains overheating - doesn't necessarily explain where the coolant is going).
Good luck with it.
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