Transmission and rear differential upgrades?

I have a 2002 Subaru Impreza Outback Sport.

It's my My, and my wife's, go-to road trip car.

I'm putting a parts list together of parts I should start collecting and replacing as needed, and I'd really like to reduce the RPM at freeway speeds. I've heard the impreza 6-speed has a VERY similar gear ratio to the 5-speed, and I should look to upgrade to a Legacy 5-speed transmission and rear dif, as the maximum gear ratio is much better than the impreza 6-speed for maximizing freeway efficiency.

Is there a particular year of Legacy 5-speed transmission that would plug in nicely to the 2.5L ouback sport engine? Has anyone done this before? I'd like to just buy the parts now and wait until the transmission or engine need to be pulled for repairs, then just make the swap then.