Assuming this is a 4.3L engine, the upper intake manifold gasket would be between the upper intake manifold and the lower intake manifold.
It is between the upper and lower intake manifold. The gasket is an o-ring type.
there is no upper intake gasket, only a front and rear
On the 4.3 engine it is located under the upper intake manifold. The upper intake manifold must be removed to gain access to it.
There are two knock sensors, they are both located under the upper intake manifold.
They are located under the upper intake manifold. The upper intake manifold must be removed to gain access to the bank of injectors, AKA the "Spider".
Assuming this is a 4.3L engine, the fuel pressure regulator is located under the upper intake manifold. The upper intake manifold must be removed to gain access to the regulator.
likely under the upper intake manifold
The thermostat is located under the elbow of the upper rad hose on top of the intake manifold.
It is inside the upper intake manifold. It is attached to the fuel injector poppets with a bracket. To inspect/replace you must remove the upper intake manifold.
The pressure regulator is located under the upper intake manifold. Find your fuel lines entering the manifold (right behind where the air enters the manifold)...the pressure regulator is on the bottom side of that cluster*%$. You will need to remove the upper manifold to reach it and change the regulator.
Upper intake manifold plenum leak/crack most likely. If its blowing out coolant then you probably have a bad head gasket.
It is located under the upper intake manifold and is attached to the bank of injector poppets, aka the "spider". The upper intake manifold has to be removed to gain access to the regulator.