OK, so I decided I would drain the system, fill it back up with prestone flush and water, drive around a bit, and then drain and fill with coolant. That was the plan.

I drained by unscrewing the rad cap, and unscrewing the drain petcock on the radiator. It took awhile but about 2 gallons came out. I removed the upper rad hose from the t-stat housing because it seemed to help the system drain.

Once it was done dripping, I put the drain plug back into the radiator and reattached the upper rad hose to the t-stat housing. I added the little bottle of prestone flush to the radiator and then filled up the rest with tap water. It took a really long time because the rad cap area would fill up quickly then slowly trickle/bubble into the radiator.

Finally, it seemed like no more water would go in, even if I squeezed the upper radiator hose to push bubbles out so I thought, OK, ready to fire her up. (edit: I also completely removed and cleaned the overflow tank and reinstalled before starting the engine.)

I started it and almost immediately, the water started rising in the radiator and spilling over the cap neck. this wasn't spilling over too fast, just a steady dribble.

Naively I thought I should put the cap on so that the water would stay in the radiator until the thermostat opened - I realize that was a bad call now. So I put the cap back on and went into the cab and revved the engine a bit to gets temps up to open the t-stat. Right at about 170 I went back and opened the rad cap and fluid exploded everywhere.

I immediately shut off the engine and cleaned up. That's how she sits now.

PLEASE - WHAT HAPPENED AND HOW DO I FIX IT?

 

That all sounds normal.
Some coolant will flow out the filler neck with cap off and engine idling.

 

Never ever loosen/remove the rad cap when the motor is hot/system is pressurized. Try idling with the rad cap off again, keeping the rad full 'til coolant/air begins to flow out the rad. When coolant/air flow out, install a new rad cap. After the motor has completely cooled (as in overnight) remove the rad cap and check coolant level in the rad. The rad should be full to the top (motor cold) and the bottle should be about 1/2 full when cold. The rad cap is critical to the proper function of the coolant recovery system.

Coolant level in both the rad and bottle should be checked (motor COLD) every day for several days to verify coolant levels and verify the rad cap is functioning correctly.