I removed the agitator and agitator coupler, but I found that the tub nut is a mess, with big chunks missing from the sides (see attached photo). It's hard to see how normal wear and tear could have done that; maybe this happened when the transmission was replaced several years ago. Anyway, a wrench can't get a grip on the nut. Can you suggest how to remove it? I guess I could skip replacing the tub seal, although it looks like the procedure wouldn't be that hard after getting the tub nut off.

One more question: Supposing I do get the nut off and replace the tub seal. Afterward, how tight would a new tub nut have to be? I'm thinking that it might be better not to have it super-tight, in case it ever has to come off again.

Thanks a lot for the help.