The thermostat on a GM 3.1/3.4 is a pita to change, you have to remove the exhaust crossover. They don't have a cap on the radiator either, so you can't use that funnel.(though they work well when you can use them)
Usually you can bleed the system out by filling the system, crack the bleeder, and start the engine. That will help burp the air.
Once coolant starts running out the bleeder, you can close it.
Then let it run and monitor the temp and coolant level, once the thermostat opens it may burp more air out.
It may keep burping air bubbles for a while, just keep an eye on it and once it stops burping you should be fine.
Unfortunately most mini vans are more difficult to bleed the coolant on than your average car, the radiator sits low and creates air traps.
1993 Chevy C1500
1999 Saturn SL2
2000 Nissan Quest
1986 Pontiac Trans Am
1940 Ford 9N Tractor