Well when I did mine my cyl heads sealing surfaces were perfect because they were new reman units. My intake manifold was original, there was slight pitting on the water port area that I cleaned up but still I used some Permatex Aviation gasket sealer on the gasket in those spots. The pre-Vortec does not have the locating pins that hold the gaskets in place like the Vortec so the gasket sealer helps hold it in place while you lower the intake.
Did mine 5 years ago, no leaks, no water in oil.
When you do yours double check that torque reading and use an appropriate torque wrench (meaning a low reading one, not a 0-150 lbs unit). I have 2, one goes up to 150 the other goes up to 75 ft lbs.
This is important because I know GM had problems with the original run of intake gaskets on the Vortec intakes on their vehicles. Antifreeze in the motor oil, etc. I actually prefer the old pre Vortec for this reason. Those intake gaskets NEVER leak. Not sure if its the gaskets, or the fact that they used 4 more bolts to hold down the intake.