Art, here's some data
Using a carb with a choke, a carb mounting pad with a B height measurement of 4" is about the highest you can go and keep the air filter below the L engine screen.
If you use a carb with the choke milled off, you should be able to stretch that at least another 0.5" higher, the limiting factor then being the height of the fuel bowl vents.
The best "dropped" air filter bases I know of are the ones for the big block Corvettes, late '60s, early '70s.
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