On an '83 model year truck, if indeed it had a stock engine, and yours doesn't (it says you swapped from a diesel in your bio.) It could also be a cracked head... Later Chevy head castings are notorious for this (believe me, I know, I'm an engine machinist.) An intake manifold gasket really won't get water into your jugs... It could be letting in excessive oil (blue smoke) making a vacuum leak (rough running, idle too fast) or letting water into your oil (makin' gravy.) Most likely your troubles are head related. If you do have the heads off HAVE THEM MAGNA-FLUXED!!!!!!!!!!!!!! And in all honesty, you should probably have them surfaced too.
If you need any more advice, PM me or e-mail me.