[dsraven]

cowl behind engine with the grille in front of windshield goes directly to the footwell vents on each side and then to a very small drain hole in the bottom of that plenum on each side. there is a fairly narrow access area below the hood on the firewall that you could put some air outlets from the engine bay that could feed into that area but the air would basically have nowhere to go except into the cab or out the grille in front of the windshield. you would possibly be better off with venting through the inner fenders.
why is the big block running so hot? does it take up too much room so the fan can't get the air to exit around the engine? how does the air flow path of the cooling air look, what do you have for direct access for cooling air to enter and exit the rad area (other stuff in the way like a big grille or driving lights or a/c and trans coolers etc), is there a fan shroud or is it electric fans?
the attached pic shows the firewall cut out and a new piece mocked up. youi can see above that sheet where the cowl basically ends and the firewall starts. the area above the sheet would be the narrow access to the area in front of the windshield where that grille is. you can also see the inner cowls have been removed for replacement and how that area looks. the next pic shows the new inner cowls and how they taper to a point with a small drain at the bottom. the drainhole would have had a small piece of sheet metal screwed over it to partially occlude the exit and leave a small opening that varmits can't get into but water could drain out. this is the reason this style of truck usually needs the inner vents, hinge pillars and floor replaced. the vent holes plug with debris and the cowls fill up on rainy days, then the water exits through the footwell vents onto the truck floor and rots things out.
hope that helps.