I'm glad you didn't blow it up!
My 66B is still positive ground, and I'm a little paranoid about it. I put a "positive earth" sticker on the battery compartment, and even added stick-on + and - signs to make sure if someone goes to jump start my car they understand the polarity.
Even so, a guy in my MG club was helping me put a new battery in it last week (I was having a voltage regulator problem and it wasn't charging), and he started to put the battery in backwards.
We're ALL idiots from time to time! You should probably change that red ground cable . . .