you don't have to fully drain it. just take it down below the top of the block level wise.

I also disconnected the hoses and used air to blow out the core before I pulled mine.
65 b body, so should be close.

It was nice because it was small enough that it came out without much trouble instead of the 66 and up.

since you think yours is bad, I might skip the blow out part.
Though I have to wonder unless they installed shut of valves for both lines, it might still be good, they just decided the old valve inside wasn't worth digging out?