I bought a 89 GMC vandura with a 5.7L engine with no muffler.IT ran o.k but was a little loud,No problem with over heating or leaks.Had a new muffler and tailpipe put on, and water began leaking from intake manifold on the drivers side.Also when i start up the engine in the morning much more then droplets of water come form the tailpipe.
This is the territory you get when you buy 21 year old beaters…
There is no way that exhaust work can cause an intake manifold to do anything.
God bless you brother that really help alot’