I have one question for you: how is a 350 in a 1998 GMC not a small block??
The knock sensor is located on top of the engine block underneath the intake.
In a 1998 Subaru Legacy, the knock sensor is located on the top of the engine block and to the left of the engine. More specifically, it is under the left rear intake manifold runner.
Knock sensor is located in the wall of the engine block facing the firewall. You have lift the car if you want to reach it.
It is screwed into the water jacket on the side of the engine block just in front of the starter. It is HARD to see. It will have an electrical connector plugged into it.
on top of the engine block under the intake manifold. it can be accessed without removing the manifold. easy fix!
Under the lower intake manifold... the entire manifold must be removed which is a 2.5hr job... and you will see it on the engine block itself.
side of block under throttle body housing.
It is located in the side of the engine block close to the starter . it has a single wire connector and it screws into the water jacket. So be careful when you remove it because engine coolant will come out
Engine block heaters were optional.
Knock sensor/vibration sensor
We will need to know which engine it has to answer that.
There would be block drains located on each side of the block very close to the engine mounts.