Assuming that the T56 you purchased is one of the mentioned factory-installed units, you'll have a few other obstacles to encounter besides the bolt pattern. Your mechanically-actuated clutch linkage will likely have to be converted to a hydraulic operation, the existing crossmember and driveshaft will probably have to be modified, and the electric speedometer output will have to be converted to mechanical operation. Because the T56's main case is so much wider than any transmission available in your GTO, you may find an extensive amount of floorpan modification is required and that shifter positioning may be limited.
A few years back, after the 467 ci in my '76 Trans Am destroyed another well-built Super T10, I began searching for more reliable manual transmission options-which included the TKO and T56. Knowing that installation of the T56 six-speed would likely require a great deal of fabrication, and that its typical maximum torque capacity was 400 to 450 lb-ft, the five-speed TKO seemed a better option. It's lighter, has a smaller main case, offers similar cruising ability, and contains high torque capacity ratings.
I searched for additional information about the TKO and found that Keisler Automotive Engineering (www.keisler-auto.com) in Knox-ville, Tennessee, was one company producing bolt-in TKO kits for many vintage GM vehicles, including my Second-Gen F-body. The kits included a TKO transmission with either 500 or 600 lb-ft torque capacities. It would not only provide reliable operation behind my 467 ci, but would expand the highway use of my Trans Am even with 3.73 gears in the rear axle. I decided that the TKO-600 kit with a 0.82 Fifth gear best fit my needs, and I documented the installation in the Nov. '05 issue of HPP.
Knowing of the company's offerings, I contacted Keisler's Jason King and asked him about the availability of a TKO kit for your GTO. King noted that, like his company's Firebird-specific kit, the A-body kit also contains virtually everything required for complete installation. This includes a modified crossmember and driveshaft, a mechanical speedometer cable, electrical wiring, and all of the associated hardware.
He added that though some floorpan cutting might be required, the company is now modifying the TKO's main case, making it slightly narrower to minimize floorpan modifications in your application. Contact Keisler directly for further details.
I feel that you'll find your idea of using a T56 manual is a sound decision that will improve durability and enhance highway cruising. I suggest that you consider a TKO five-speed as well. Though the T56 offers an additional Overdrive gear, that benefit may be outweighed by the installation modifications required. If your T56 happens not to be an aftermarket GM T56, the installation may tax your skill and patience levels.
The TKO, on the other hand, contains only one overdriven gear, but it could be easier to install, especially when you consider that complete installation kits from Keisler or a similar company are available. Whichever transmission you decide is right for you and your application is ultimately your choice, but I strongly believe that either one will greatly reduce the longevity issues you've experienced with manual transmissions!
Q-Jet Quandry
I am a new subscriber and I have a question about the replacement of a Holley dual-feed carb that is on my '69 Firebird 400 with R/A-IV heads and cam. The Holley is too big and/or jetted too rich; the exhaust stinks and it really sucks fuel fast.
I want to go back to a Q-jet, and I remember being able to buy a brand-new R/A-IV Q-jet not that long ago in a YearOne-type catalog. This doesn't seem to be available anymore, and I have looked in a number of places for it. Can you tell me if I can get a new one anywhere?