Thanks for thinking along. Unfortunately I believe I cannot test it like that. Old mercruisers like mine have a very simple system for crankcase ventilation, actually it does not even have a ventilation, never mind a PCV system. Its just two open hoses coming from the valve covers that breathe out foul air that is vented close to the flame arrestor, then sucked in by the carb. The air actually does not get into the crankcase from anywhere, all that comes out is just the blowby from the cylinders. When disconnecting the hoses both blow noticable amount of air/condensation/blowby out. The blowby and positive pressure is unpleasantly noticable since the engines are not even broken in. This is on both engines similar.
But,your idea is an interesting one. Since the ventilation works on blowby pressure from the cylinders, which is rather considerable so a small vacuum leak would still suck oil in and still have a positive pressure coming out of the valve cover holes. So I think your theory is possible, but I think I cannot test it in any way without pulling the manifold and inspecting the seal surface - which I will probably do anyway when pulling the engine.
But I will play around tomorrow a bit, before taking the boat out.

Thanks!

Actually you can. Your set up is even simpler because you dont have to worry about the pcv system. You would just block of one of the valve cover holes and do the test on the other one BUT ........ if you have a bunch of "foul air" (blowby) coming out of the holes then you probably dont have an intake leak. If the intake was leaking it would be pulling that foul air into the cylinders instead of it going out of the valve covers.

To be honest with you if you have a bunch of blowby from the valve cover holes then something is wrong, especially on new engines. All engines will have a little of it but a fresh engine should have very very little. It sounds as if the piston rings did not seal properly or are installed wrong.

Well, the mechanics convinced me that there is time needed for the rings to settle and I should give it some time. Since I haven't been able to run the engines at all after rebuilding, they have been working one or two hours max so I cannot confirm or deny their argument. But the boat is out now, and we´ll start pulling the engine in a week or so and lets see what they find out.
The strangest part is - today it ran better then on previous days. I could not get it to smoke at all. Ok, it was really windy so hard to notice but this is extremely mysterious. Since everything external has been checked, I hope the internals bring some clarity.
The engine guys suspected either a micro-crack in the heads that opens when warmed, or something moving around in the valve area... lets see.
https://cimg8.ibsrv.net/gimg/www.off...87504c0555.jpg

Modern piston rings are broken in almost immediately if the cylinder walls were finished properly. If you bought a new boat or new engine would you expect it to smoke and burn oil ? I dont think so. If you have run them 2 hours and the rings are not sealed then something is wrong. But you dont know if the rings are the problem.

time for a leak down test.