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I had one on my 71 blazer for the 11 years i owned it & drove it everyday.
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how long have these filter tops been on the market? seems like they just came out about 5 or so years ago.
it does seem like a good idea at first. but from what i learned in my engine design classes in engineering school .....the air speed in the venturi's will approach Mach 1 at WOT. the carbs are designed for the 90 degree turn of the incoming air into the carb, hence the rounded shape of the upper venturi. when you combine the air coming in at 90 deg's, with the air coming in straight down, you drastically change the flow characteristics especially approaching WOT, ie mach 1.
the atomization of the carbs were not designed for this funky flow of air. not to mention pulling the hot air out of the top of your engine compartment at low speed or takeoff. in my opinion, you would want either straight down, or 90 deg's, not both.
remeber the air bleed sit in the throat of the carb. if you pass turbulent air across those air bleeds, or if you pull air straight down, you screw up the vacuum signal in the carb. they were designed for a steady flow of air across the air bleeds to providing stable vacuum to pull the fuel through the fuel passages.
i have never run an open top like what is being discussed here, but i did run an edelbrock cage type filter, which pulls from all direction. i noticed a loss in power and torque, plus some backfiring when i installed it. it all went away when i took it off and went back to a round style with a solid top.
for what its worth, on a street engine the old stock dual snorkel type intakes that these trucks came with would be the best bet. it pulls the cool incoming air from the fenders and keeps a uniform air flow coming into the carb. its a differnt story on a healthy race engine.