Here's my how-to on installing an SG shaft in the GT.
Note: I used a traxxas clutch on my truck, if you use a different clutch this procedure might be different.
First of all you need to drill new holes in your engine mount and tap them with a 4-40 thread. The holes basically have to be drilled as close as you can get them to the original holes. I think mine are 5mm behind the orginals. If you don't want to do this you can buy the Klien motorsports SG shaft engine mount, which is quite a bit stronger as well, but its also expensive. You can also see where I had to remove some material at the back of the mount so my back plate screws didnt hit the mount. You can actually get away with using the stock holes, but its not possible to back up the traxxas clutch bell enough to get full contact with the spur gear, you get about 2/3rds contact, so you could do this with a steel spur.
Now I used a Tmaxx flywheel for this, which I then got a buddy of mine to turn down on a lathe so it wouldnt protrude out of the chassis. Traxxas just came out with a non-pull flywheel for the Jato that should work perfect for this. To set up the clutch all that has to be done is the bevelled cone needs to be spaced out about 2mm, and the clutch nut must be ground down so that it does not extend past the threads on the shaft at all. I also had to grind te corners of the hex on the nut a bit so the clutch shoes weren't pushed out at all.
After putting one shim on the shaft so the clutch bell wouldnt rub on the flywheel I broke the outer race off an old 5mm bearing that was 4mm wide and used the inner race as a spacer. Then I just used a cap head 3mm screw to hold it all on.
If anyone knows of a traxxas clutch bell smaller than 16 teeth please let me know so I can get the normal gearing back. 16 teeth isnt to bad, but for a short track it makes the engines power band a bit weird. Luckily my engine is really powerful anyways, so the taller gearing doesnt take away much from the low end snap.
1xOFNA10098 SG clutch nut
1xOFNA10099 shim kit
1xTRX4146X clutch shoes+spring (high stall)
1xMIP1602 traxxas clutch set
1xTRX4116 16t clutch bell
1xTRX3281 Cone split beveled
1xTRX4142R Flywheel for Jato non-pull (Not what I used, but this is essentially what I've got now)
1xTRX5114 5x8x2.5mm bearings
1x 3x10mm cap head screw