I was drunk during the last post so dont blame me.
You should be able to reason with the parts manager regarding the wrong seal.
I would be more concerned about the hosing they're giving you on the driveshaft. To shorten a driveshaft should only cost about $65.00. Lenghening is a little more about $85.00 at a reputable driveshaft shop.

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Nothing in this world can take the place of persistence. Talent will not; nothing is more common than unsuccessful people with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. The slogan "press on" has solved and always will solve the problems of the human race.
Calvin Coolidge (1872 - 1933)