Kerry, There is no need of "back pressure" to allow acceptable running in your case of limited use----for fine tuning, yes the engine will respond slightly to changes in exhaust restriction and shape.
There should be no need to seal the heat stove area to prevent vacuum leaks if the intake manifold is sound.
I think you are right in suspecting a manifold to head leak.
Running a stream of water over the suspected area can show a leak if you don't let it ingest too much.
