Other than to resolve a leak, I've never seen a recommendation to bypass a squarebody heater core. They are usually continuous flow (always have water running through ), so there isn't much of anything to fail except to start leaking.
One very common heater problem is that a lot of junk falls down the defroster vent and ends up in the plenum behind the glove compartment. If you remove the glove compartment door you can access the screws that hold the panel on the heater plenum right behind the glove compartment. Then you can see the blend door, move the controls to make sure it goes full travel, and pull out the pens, pencils, matchbooks, parts receipts and other junk that has fallen down there. You might even find remnants of a mouse nest they made from the fluffy insulation on the firewall.
Bruce