Head / Block Leak (Possibly)

[Steve Jones]

Agreed. And tightening the plugs down to 30 lbf.ft seems an awful lot. Hand tight and a good nip seems a lot more reasonable.

Steve

[Charles P]

I would check that the steel plate on the exhaust manifold is absolutely flat. It may have distorted slightly when the exhaust pipes were welded on.  With this type of manifold set-up there is quite a lot of strain on the fixings into the block, unless they are good and tight you could get leakages.  If I was getting a hissing noise my first instinct would be to look at the plugs.   Have you tried turning the engine over in neutral on the starting handle (with the ignition off!) and listening carefully to the spark plugs?

Putting a bit of water in the plug wells on a cold engine helps to identify bubbling at the plugs. If it's a '37 head bored out for 18mm plugs, the washer may not sit flat due to ropey machining. 
Steel manifolds are frequently a bit bowed and I find that my manifold bolts do loosen over time unless tweaked.

[John Mason]

If the hissing sound is near to where the top water outlet is check the hose clips. A small water leak dripping from hose onto a warm head will make this noise as it boils and evaporates . The head is the first part of the engine to warm up.

John Mason.