As of recently I am the proud owner of 1967 buick gs. Not knowing everything about it I have found good information here but not everything I am looking for.Firstly the car seems to be in pretty good condition and I am asking for suggestions or information that would help me with diagnosis of its currrent symptoms.
I have no history on the car but when I first got the car there was excessive water coming through the left tailpipe. Shortly thereafter I noticed the radiator fluid was being drained when we would sit and let it idle for the first few days. After it warmed up this problem was solved and hasn't occured since.
After doing the once over the next thing I did was a compression check. All cylinders seem to be hitting at 180psi except the 3rd one back one the left side hitting 100. Most the time you can feel it missing when putting your hand at the tailpipe. Both sides have cracked exhaust manifolds and I noticed with the cylinder on lower compression there is a large manifold crack directly below. The plugs when pulled seemed to have a white coating on them as well.
With my current knowledge I am not able to even begin with a particular diagnosis. Anyone have any clue? Dumping oil in the cylinder did not seem to have an impact. Could this be something as simple as timing or worn valves? Before taking it to get examined or doing it myself I wanted to grab feedback.
I'm also curious where people have mounted temperature gauges, haven't found a spot yet. Lastly converting to a manual transmission and any info there.

losing water into the exhaust system is almost always a bad head gasket. your drivers side compression issues reaffirm that.

there is no way for the water to get directly into the exhaust without first passing through the combustion chamber unless the head itself is severely cracked.

When you change the gasket, you should check the head is straight. Take it to a machine shop and have them place a straight edge on it.

Much thanks. I figured this was the direction I would be taking.

It is also entirely possible, but not absolutely certain that this engine may have been overheated badly at one point in time or another. If you are going to the trouble of pulling one head go ahead and pull the other one too. If you have a head cracked due to overheating there may be other problematic areas as well. Check for cylinder wall scuffing and scoring. If there are signs of bad wear (enlist the eyes of a friend who knows the signs if you do not) the entire motor will likely need a rebuild from ground up. Overheating can cause oil to dilute and you may also notice evidence of coolant in your crankcase oil. If thats the case, the rod and crank/cam bearings will likely have been damaged as well. Good luck.

As the new owner of Brett's 67 GS-340 CS (thanks for the deal Brett!!)... I have a question too. If the low compression of #6 where due to a bad head gasket, then wouldn't #4 or #8 have have equally low compression too? If not, where is the compression volume going? My guess is a burnt valve.
Regarding future plans... I just got the parts to do the disc brake conversion to the single piston calipers, I'm still looking for a disc brake master cylinder. I was woundering if I should also replace the proportioning valve... are they brake type specific?
I just found a cherry 3-speed console w/ the tach hole, but no tach. However, it does have the factory blank.
I'm looking for a BOP TH350 or 700R4, also GM strato bucket seats.
I know I'm potentionaly devaluating this rare car... but I will be keeping all the parts including the front bench seat and the ST300 trans, but I'm not sure if I should keep the drum brake parts.
Thanks for any comments?
-Drew

I Forgot to mention my plans for the engine.
The exhaust manifolds will soon be comming off for repair. After I eliminate the exhaust leaks I'll be able to ID any valve train or lowend noise. Depending on that outcome the engine maybe coming out for a rebuild. I'm thinking that it may not need a complete rebuild because of the good compression from the other 7 cylinders, and if so this engine must have had a rebuild over the last few years, the odometer indicates only 1K miles.
What should the compression be on a fresh 340 w/ the factory cam?
Thanks again,

-Drew