[gnx7]

Here is why you want the T56 Magnum.....

The 2.66 version is basically a close ratio 5th speed (.80 5th) with .63 6th gear. Truly the gearing is primo. I would like a little more overdrive... but oh well.
They hold 1000rwhp in stock from. I'm pushing over 1400hp with my 100% stock unit shifting at 7800rpm in 1-5th gears without ANY issues running a stock slave cylinder and RXT twin disc. 5th gear is holding about 1400hp without issue. This is in a lightweight 2900lbs car though (LS9 turbo RX-7).
It modernizes the car with how smooth they shift once broken in. That alone is worth the cost.
You can change out 5th/6th gear if you want ZR1 gearing or other ratios....
Abundance of replacement parts in stock (older T56's have parts backorder issues)

The TKO600 was designed many moons ago and doesn't shift well at high RPMs.
4th is 1:1 and opting for the .64 5th gear the RPMs fall off a cliff
Shifting smoothness is not even in the same ball park as a T56 Magnum

It is much smaller though and about 50lbs lighter as an attribute.

Gearing will depend upon how much power you plan to make... and redline....
here is a good calculator to play with to figure it all out
http://fitbodyshop.net/gear/index.php