I've got an Espar heater Henry installed with the build in '05. It pulls air from under the dash, and blows it down at my feet.
I'd strongly recommend not doing it like that.
What happens is that all the heat gets trapped under the dash, and sucked into the heater. There are sensors upon sensors which use any and all data to shut the unit down if it perceives there's any reason whatsoever to do so. Warm/hot return air (supplied to the heater) is enough to shut the burner down. Just about the time it get's crankin', down she goes.
I'd suspect it'd be the same thing if you did it the old-school way with the heater boxes. An old heater would be fine, but a new solid-state controlled Espar won't like it.
Sorry.