In my recently acquired '81 C3 I am replacing the very tired L81 with a GMPP 350/330HP vortec long block.  With a few modificatons everything is working just fine -- with one exception.  The GM dual plane intake manifold I purchased to match the vortec heads is nearly one inch higher than the intake on the L81.  All indications are that the stock air cleaner atop the quadrajet will not clear the lowered hood.  Have any of you run into this problem?  If so, how did you remedy it?  If possible I would like to keep the stock air cleaner.  Thanks.  

Try a K&N filter assembly.  They make a drop bottom plate with a short element and a filter cover/top plate.