I too like the clean under dash look, those add on vents just look wrong if you want an original appearing truck.
I'm planning on Vintage air and make or find my own vents to suit, through the speaker grill probably.

I don't think vent location is so tricky you can't mix and match to suit, pick the brand you want and figure out vents you like.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-