The RPM drop from fourth to fifth with a .64 OD makes it all but unusable for anything. Most Cobra engines have bad manners when asked to operate below 2000 RPM. They tend to buck, chug and fart because they work better in the midrange. The RPM drop from fourth to fifth with the .82 OD makes it feel like a real fifth gear and gets away from the bucking and farting.
There is a bigger issue yet you may want to ponder. The TKO transmission is a direct decendant of the 3550. Some internal gear design changes improved steels and presto 500 and 600 lbs-ft torque capacity. The transmission's weakspot is a mainshaft/countershaft misalingment on third gear that is 0.140" (or more on some). The misalignment leads to a premature third gear failure if you are hooked up and hit third with the kind of suds a 500 HP engine can produce.
You may hear a lot of that won't happen to you kind of talk but the more significant statement might be why put yourself in harm's way when you don't have to? So what's the alternative?
The T-56 is, not withstanding the published specs, a superior unit from both a design and shifting standpoint. The shifts are much smoother and cleaner because of the basic design but also because of the triple cone synchronizers used in the unit that are not available in the TKO. Additionally there are significant aftermarket gearsets available that had their genesis in factory backed race efforts. These previously unobtanium based pieces can raise the transmission's power transmission limits to a reportedly 1200HP and 1000 lbs-ft of torque level!
Are the numbers real? Don't know. Lets say the real numbers are 25% lower; you're still at 900 HP and 750 lbs-ft of torque! Additionally you have very nice fifth and sixth gears with .89 and .75/.76 ODs in fifth and sixth. The fifth and sixth gear changes actually feel good and the engine doesn't take a big gulp as it drops to near idle rpm on the gear change like a 1:1 going to a .64OD.
The transmission is bigger than a TKO and about 20# heavier than a TKO but it will fit in the car with a little bit of the same elbow grease we used to put everything else in the car. Here is the link for the Transzilla option ==>
http://www.rsgear.com/transzilla.asp These are basically the previously unobtainable gear sets GM and Chrylser used in their Corvette and Viper factory race cars. They's potent!
Attached is a pic of a broken TKO600 third gear. It ain't pretty.
I'm having difficulty uploading the image. You can read the whole story (and see the pictures) on CC here ==>
Tko600 Warning!!!!
If you haven't purchased yet give serious consideratioin to the Transzilla offering. It'l fit and it is impressive!
Ed
edited because I can't spell worth a damn