Forget the heater, you can get universal units to replace it.

Then you can put whatever you want in it.

I'd ditch the 3.9 liter engine idea, that's about the most craptastic engine you could pick. Seriously. They are still dog slow and the mileage sucks. It may look good on paper but they just plain suck. Even Poly 318s make fun of the 3.9.

I'd go 360 or 440.