According to IVA, air cannot be drawn into the heater from the engine bay. This means you must seal the heater somehow and then have ports to draw air from cockpit.
I wanted something good looking and not to just stick some plastic ports over the heater and call it good.
Therefore I welded up an aluminium box after ordering the parts pre cut. NOTE: Flange has been redesigned and angled to mate with body and to allow box to protrude into dash area a little more., see pics.
I made sure this was water tight, and then made required holes and added nice ports and metal corrugated hoses into cockpit.
I will add some gauze etc over these holes in the cockpit to stop mice from nesting!!!
Again there was some unknown as you need to fit this box before you mate the body with chassis. Will it all fit, or will it hit the bell housing.
I measure best I can and I have couple of inches clearance. If I was to do it again, I would make a tighter fitting box or even have it protruding into cockpit another inch as quite alot of space behind the dash.
I need to cut out the holes in the plenum (plastic front) to go to demist holes and maybe to dash, pointing down to keep your toes toasty.