1987 Jeep Cherokee 4.0.

Just replaced the thermostat housing and thermostat (195). Flushed the system and refilled 50/50. After letting it idol for a while the overflow tank was completely full and pushing out the cap. I only put two gallons in the system. It's also a new pressure bottle. This is a closed coolant system (no radiator cap.)

 

Most likely you need to bleed the cooling system properly to get all of the air out of the system. Air in the system will cause coolant to boil, expand, and come out of the bottle like what you're seeing.

Plenty of tips and tricks on the forums how to do this properly including drilling 2 holes in the thermostat to bleed the head better, removing the coolant sensor from the back of the head, but I usually just:
1. Get the Jeep on a decently pitched decline
2. Turn on the Jeep, coolant cap off. I put a dime in between the idle stop and the throttle body so it revs around 12-1400RPM .
3. Heater on hot, fan on full, let it idle until the thermostat opens, put the cap on, and you're good to go.

I should ask though, why were you replacing the thermostat housing/ tstat in the first place? More than likely it's time to do a thorough coolant system overhaul, which just takes time more than money.

 

The cap could be losing pressure also. Get a 703-1396 cap from Napa

 

Likely to be a leaking cap.

When I first start an engine after having drained the coolant I check the heater hoses to verify the coolant is circulating. If they don't start to warm up within 30 seconds or so the pump is probably air locked. If the vehicle has a heater valve in the hose it has to be open; heater control on 'hot' or manually open the valve.