Re: 6" Extended cowl for a 84 Mercury Capri!
The extended height cowl is for clearance with tall intakes. I believe the carb is supposed to take air from the opening on the back end of the cowl extension, at the base of the windshield, to get cold, low pressure air. I'd think you'd want to make sure the plenum space under the cowl is sealed off from the rest of the engine compartment so the crab is only taking in air from the cowl, just make sure the cowl has proper size opening in it to pass enough air.
1970 XL, 351W originally 2V, now 4V
670 Street Avenger, Edlebrock Performer intake, Comp XE262H, Flowtech Headers