i own a 1978 datsun 520 with l20b engine and 3 speed automatic tranny. i want to swap the tranny with a 5 speed manual from a 1978 280z. what i need to know is will it fit? im pretty sure it will, but want to be sure b4 i buy it. i know the engine is swappable, so i would assume the tranny's could too

http://dimequarterly.tierranet.com/tech/transmissions.html#flywheel

This should answer all your questions.

Let's see...

i own a 1978 datsun 520 with l20b engine and 3 speed automatic tranny.

No such vehicle exists. Did you mean 510?

i want to swap the tranny with a 5 speed manual from a 1978 280z. what i need to know is will it fit?

Of course it will.

i know the engine is swappable

No, not really. It's a bit long for a 4 cylinder vehicle.