Is there a spray sealant for air intake leaks?
Not that I've heard of. Replacing the gaskets and inspecting the manifold for warping or cracks is the best way to stop intake vacuum leaks.
Can the Intake Manifold Gasket cause a car to stall RPM shifts up and down and when in a complete stop the engine wants to go into gear?
check the air compressor first to see if its working, if its working then check the air lines that connect to the air struts, if those are good, that check the air strut its self to make sure there are no leaks even if you have to spray water or something to see the leaks, if any. it has to be one of the three.
Because a true 350 vortec I think relies on the vortec intake in order for the pressure to stay correct. I just remember i had my vortec box off of my truck one time I started it and It died just put the old vortec air intake back on and it should be fine. sounds like you have vacumn leaks around the intake manifold, this can be checked with a can of carb spray,