Two cents...
 

I chose the TKO over a 6-speed also.

I had 3.89's with my 5-speed and highway driving was very comfortable with one overdrive. But final drive wasn't an issue, it was 1st gear. The 5 and 6 speed transmissions share the same 1st gear: 3.27:1. After my first drive I immediately yanked the 3.89's and swapped in 3.25's.

Frankly, the combination of a 3.27 1st gear and 3.89 rear gears is bad. Read my "Driving Impressions" thread. With the engine you speak of you won't need the torque mulitplication of a steep gear, anyway. So why suffer with an ultra-granny first gear?

IMHO, putting in a rear gear that matches the 3.27 first gear would result in a 6th gear cruise rpm just above idle, and that's just no fun.

Correct me if I'm wrong here but the only T56 with a 3.27 first gear was 1993? Later years have a taller first gear. My Viper T56 has a 2.66 first gear.
Also,trying to run a 3.25 rear gear with a 6spd might be a little frustrating as the final drive ratio would be so tall,you would rarely be able to use 6th gear.
FWIW,I'm running a 4.11 cog and am only turning 2,000rpm in 6th gear at 70mph,with 26" tall tires (335/35/17 Kumhos).

The Z06 and GTO also come with a 2.97

http://www.rjsmith.com/z06.html
http://www.newagegto.com/05GTO.html

Some 93 f-bodies came with a 3.36
http://www.f-body.org/oldfaq/html/faq/index.html

I know it can't be done, but not sure why.

Why can't you put a late model one piece rear main flywheel, on a old style two piece rear main crank?

Mark, this isn't my strong suit, but I believe a 2 piece rear main seal crank has a 2.5" register and a 1 piece rear main seal crank has a 2" register.

I stand corrected. For some reason I always thought they had the same steep 1st gear of 3.27:1.

Well, with that in mind, I guess you can go a bit steeper in the rear with the 6-speed than I thought. Have fun!!