Yeah I would say the difference between the heads would be much of a muchness. If the factory installed heater uses the Starboard side feed off the head, then I might just go with that.
Re the core, I live in New Zealand, so Im looking at around $1100 for a heatercraft one once shipping is added etc and I kind of enjoy making things. I have a tig welder so will build a enclosure out of alloy and a old truck/car heater core. I can get access to heaps of cooling fans from servers/network switches from work so hope to use a few of them and end up spending somewhere around $2-300 for the whole thing. If I cant find a fan/s that will move enough air, I might look into buying a automotive blower.