I have water in #6. We had the heads done. Pressured checked the exhaust manifolds, risers and elbows. Replaced all gaskests. Checked #6 for any sign of a crack couldn't see any. If we start the engine and run it for a few minunites then shut it off # 6 fills with water. Cracked block or maybe a cracked intake manifold ?? Also put the pressure gage on #6 but it holds very little pressure.

Sounds like a cracked block or warped head(s).
The intake manafold has no water passages in that area, only at the front and rear ends are there water crossovers.

Check valve springs. If you have a leak down tester use it. Could have warped exhaust valve. Which would drop cylinder pressure and pull in water.

just went through it,definetly a split cyl......we accually had to tear the engine down and bored the cyl which exposed a small hole in #4 cyl.....pull the piston out and put a whone in it if its a big crack the metal shavings will accually stick to the crack...this little rust spot turned out to be a dam whole,pressure tested everything,this was even a late model 330HP motor from GM.back in the late 80's GM used canadian casted motors,inwhich there was no real problem,until a few years go by.....turns out the castings were desighned to be thinner....lighter weight maybe,who knows!!!!!!!10 years old? you want to question the castings......