When I did mine recently I removed the housing from the block completely and pried the old seal out, which actually put a scratch in the housing. So I ordered a new housing along with a new seal. It fit flush, so I'm not sure why the one they gave you didn't but you got it figured out so it sounds like you're good to go.
Also, by removing the housing from the block entirely, it allows you to put brand new RTV in there, too. Might as well go all the way while you have the dang thing apart, right?