You’re learning one of the first lessons of auto work. You only have a small problem. Drain the radiator and add 100% antifreeze. Only after you drain the radiator. you might pull one of the block drains too. On the old 350 engines, you would sometimes have to poke a screwdriver into the block drain hole to punch through the blockage which seemed to be a thin wall of iron, (it didn’t seem like residue). One other trick to do instead of pulling a block drain is to disconnect the lower radiator hose. That would sometimes get additional coolant out. You might end up with some extra antifreeze by the time you get a 50% mixture in the engine, but that’s not so big of a deal. You can use it to add to somebody else’s radiator. 50% antifreeze is what you fill up with if you don’t flush the radiator. You got experience and it didn’t even cost $20.