If you have a lot of issues purging air from the coolant on these vehicles there is a good chance you have a lot of schmoo built up in the rad/block. The more gunked up they are the harder it gets to bleed air from the system its also more likely that you don't have coolant running through the entire rad because paths in the rad get blocked by the gunk. If you've just done a basic flush and the system doesn't want to fill/bleed properly I suggest getting a kit that hooks up to the garden hose to flush out the entire system along with some additives to help remove the gunk during that more advanced flush. flush the block and radiator separate if you're using the garden hose kit as this will insure less gunk from the block gets lodged into the rad, sometimes it gets to the point where its better to buy a new rad... if the heater core busts and you have problems bleeding the system after bypassing or getting a new one that is a fairly big sign that you have debris in the cooling system. when refilling the system I suggest getting the mopar HOAT orange coolant that mopar is now suggesting for the magnum engines, it helps keep the system cleaner than the old green IAT.

(ps when filling and bleeding the system on these vehicles you should be getting the vehicle to operating temperature so the thermostat opens, bleeder valves and such on the T-stat really don't do as much on vehicles like this they're more for vehicles that have a more complex bleed procedure/cooling system, you really shouldn't be having problems bleeding the air from these vehicles and if you do it usually is an indication of another problem you'll have to deal with sooner or later.

[Edit]: Make sure you flush the system with distilled water after flushing it with the hose, this will help prevent a lot of issues especially if you have hard water in your area.