Mike, that is essentially how the factory installed VW Bus gas heater worked - the engine heater boxes outputted right to a blower that outputted into the gas heater input. Don’t feel it is warm enough with just engine heat? Turn on the gas heater from the dash and the heat would drive you out. No new ducting needed. The buses I’ve seen had a BN/BA-6 heater (a BIG bugger) which was hung amidships under the floor and just sat in the middle of the heat ducts.