Replacing a leaky heater core (which is just a mini radiator, really) has traditionally been a pain for 50+ years. This is because the HVAC equipment is in the wrong place: crammed under the dash.
Take a look at early car A/C installations. They mounted all that junk under the rear parcel shelf or un-used areas of the trunk. That is where all that crap should go; easy access, better weight distribution, better occupant impact safety, fewer specialized parts for different vehicles, and it'll melt the snow off your rear window like a motherfucker!