GM-GMC: water in oil, oil filler cap, head gasket problem

Question
I have a 1885 G20 Conversion Van with a 350,4bbl with AT.  It runs rough and keeps losing water.  The exhaust appears to be like steam when it is not running rich.  It is a fixer upper but I am concerned about the loss of water.  I think it is going into the oil pan.  I have had a engine wiht a cracked cyl before but it does not seem to be that bad.  Is it possible I have a head gasket problem, and if so how do I check it out.  Compression test?  This is a 350 gas engine and I know your expertise is diesel but the basics are the same I believe.  I hope you can help.  Thanks.

Answer
Yes, it is very likely a head gasket or cylinder head crack.

A compression test may or may not reveal the problem.  

If the coolant is going into the oil, you see should a rise in oil level, particularly overnight, and the oil may appear foamy on the dipstick or just inside the oil filler cap.

You might want to try a product called Bar's Leaks which is made from crushed walnut shell and ginger root.  It might stop or slow the coolant loss.  I have used it on a GM diesel that was leaking coolant through a cylinder head stud.  You want the brown liquid with goopy looking sediment in it.