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
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Project: 1969 Chevelle, alum 5.3, GTS76 turbo, Chiseled Perf A/W intercooler, Holley Dominator EFI, Davis TC, JRi coilovers, 14" brakes, T56 Magnum. CTS-V Recaro seats!
Coffey Fabrication Nashville, TN doing the fabwork! 800rwhp+/25mpg+/9's@150mph goal
https://lateral-g.net/forums/showthread.php4?t=52976
1993 Mazda FD RX-7, built LS7, T56 Magnum, Holley HP, Davis Tech TC, 8.8" 3.73 Cobra IRS diff, DSS axles, Ohlins coilovers, big brakes.
CCW 18x11 front 315/30/18's and 18x12.5 rear on 335/30/18 RA1's,
2850lbs
10.31@137mph 165mph Shift-Sector 1/2 mile speed
441ci AllPro headed motor and 200lb weigh reduction in the works....