Contrary to popular belief having a 6th speed doesn't effect your highway cruise RPMs. My buddy just went through all this and found that 6th gear is almost the exact same gear ratio as a T5, and 1st through 4th are almost the same too...they basically just added a gear between 4th and 5th to add a 6th gear. You should be fine with whatever you gear you would normally run...probably a 3.73 or 4.10.
Don't go too high with the rear gears. The 6-speed has more gears, closer together, and if your rear end ratio is too high, you'll have to shift through them very quickly.
So pick a gear that'll get you up into the power band relatively quickly -- maybe 3.55s. The closer gear ratios will keep you higher up in the power band each time you shift. That's where the advantage is.
The 5th gear ratio is easy to change in the stock transmission "t5". 5th gear is independent of the rest the gears. You just pull the tail housing to get at 5th.
There are aftermarket 5th gears for the T-5 that are a lower ratio for highway cruising in 5th gear.
Of course, i've been wanting to do a T-56 swap myself...just for the sake of a MUCH stronger transmission and just to say i have 6 gears
Like said before, there are different versions of the T-56 with different gear ratios. You gotta put a little thought into which will work best for the HP your engine is putting out
I did see one T-56 setup that had 1st - 5th gear ratios that were essentially the same as a stock T-5, plus a 0.50:1 6th gear. That would probably be great with 4.10s. You'd have scorching 1st - 4th performance, while 5th and 6th would be great for driveablility.
Another thing i didn't like was how close the gears were. That might sound stupid since most people want that, but i was constantly going into the wrong gear. There's very little left to right movement needed at all to change gears. I guess that's good for some people but i hated it. For the price i'd just got a TKO 600.