Manifold insulation for 300K

["Thomas L. DeBusk" <cfdebusk@xxxx>]

I was fiddling with my (new to me) 300K yesterday, doing some cleaning 
and trying to get it started. I discovered underneath the intake 
manifold at the rear a piece of insulation covered by foil. Is this a 
very rare and coveted option? If so, what is it called?

If not, why did somebody stuff this under the manifold? I suppose it's 
to keep heat either locked in, or away from something?

Incidentally, either I have a very stuck valve, or it needs some major 
engine work. While cranking, it developed a very solid mechanical 
"thunk". It won't turn past that point. Tried it several times by hand. 
Hopefully it's a stuck valve. If not, I'm in deep animal waste, I think. 
Took the valve covers off and discovered several of the valves have lots 
of free play in the rocker arms, so I'm getting the picture the valves 
are not moving too happily. I soaked with Blaster, and I'll try cranking 
again after a little while. Other suggestions are welcome.

Thomas