Or go to Napa & get p/n 6603100. Seems like it's from an 80s Mustang. Should cost like $25.
And to make it not happen again, setup your heater hoses with either radiator flush Ts (so you just remove the caps & it drains) or garden hose fittings (like your exhaust manifolds). Makes it easy to drain the heater core in a few seconds.