You can run a 14" after market air cleaner on a Quadrajet as long as you are using a flat base air cleaner. (I believe the factory diesel air filter for a 6.2 is a 14"x4".) If not, you will need about a 2" spacer. On my '79 I used a factory air cleaner base, but switched to the taller 80's era lid. It is 2" taller than the factory lid for 73-80. You will need the correct filter to go with it. I purchased my snorkel through LMC Truck.
FYI, my truck ran better with this set up in hot weather than the open element air cleaner. I did have to adjust when the secondaries would open though. If you look at the LMC diagram, I cut off the end cap of the inlet cover. I did get some more bugs in the air filter, but it gives you more of a ram air set up.
http://www.lmctruck.com/icatalog/cc/full.aspx?Page=162
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