You might be through this already but a couple of note,
The heater core in these is full flow, there is no water valve. The hose fittings are on the top tank and the core is a double pass.
This means the air will get out easily but the lower tank of the heater core is hard to flush crap out of.

I get the impression this truck ran with straight water in the cooling system at some point in its life. Makes a mess in there. I have no clue why vehicles down South get run without coolant or at least a rust inhibitor added to the system. Monteros will get rust through the water pipe under the intake pretty quick.
If all the sludge was from mixing coolant, I guess that happens at times. PITA.
When I lived in FLA we used distilled water in the systems since the city and well water is harsh on cooling systems.

I read that you filled the system with the bleeder open, closed the bleeder before placing the radiator cap on. This defeats the purpose of the bleeder. You want to close the radiator cap, start the engine and crack the bleeder open a bit to vent the high spot. If you open the bleed on a hot engine it will blow off the pressure, might get you when doing so. But during cool down the system will refill and will pressurize the next run session.
One should not open the radiator cap to take a look inside. This can at times let a gulp of air in that needs to be bled out again.
As long as the coolant bottle has fluid in it and the system is not leaking the system will be full. Even most systems with small leaks will draw from the bottle rather than drawing air back in the leak.
Yes, there are exceptions to all this.