Try to undo both of the heater hoses on the firewall and connect a garden hose to circulate the funk before you try to change the heater core. I had to come put this back together for a friend who wasn't aware that this is what he was getting himself into

 

wow..that is gonna be fun to put back together...good advice

 

its all back together...not saying there weren't screws left over but its together lol