I rebuilt my heaterboxes with larger diameter J-tubes with fins welded to them on the inside, and bypassed the motor fan altogether so it drew fresh air from the interior instead of the stinky engine compartment. Here you can see the tube on the outside, it's the only pic I have of it. Where it heads up to the engine compartment I cut it off and welded a plate over it, the tube you see has a bend in it that mates with the side of the heater box at the back. The air comes from the interior via a bus fan, into this tube, then continues the rest of the flow as normal. I had to find a decent(not to rusty) heater box, bend back all the flanges, remove the original J tube with aluminum(magnesium?) heater core, welded some fins on a larger diameter J tube, and then welded everything back up again. Best of both worlds, no stinky engine compartment air. If you lived in a really cold environment, I'm not sure you'd get adequate heat for a really cold day, but for most places it would be fine. The air was hot coming out and with the fan, the front window actually stayed clear.
Jason