The pictures fooled me. I measured the countershaft gear at 23.5mm wide (0.925")in the photo and the mainshaft gear at 28mm wide(1.10").
If we use your measurements and assume the wider gear was the correct design width then the .975" wide gear has only .835" of face width in contact with its mating gear. The transmission's torque capacity in third gear would then be (0.835/1.01)*600 or 496 lbs-ft.
I used 600 lbs-ft as the transmission torque capacity because that is what Tremec advertises on their website
http://www.tremec.com/English/products/TKO.asp
If you have the tires hooked up and an engine with approximately that much torque and then add the rotating inertia of the engine-flywheel-clutch assembly at say a 6000 RPM shift point, it is not hard to see how you could induce a cascading third gear tooth failure that would get both the driving and driven gears.
I believe that even though you have good replacement parts you will desroy them yet again if you engage third gear under power with the tires hooked up. The gears need to mate up without misalignment and should be the same width. Look at all the other gear pairs, that is the way third should look.
This is minimally a manufacturing QA problem if the wrong parts got put together. If these are the right parts then we have a much bigger problem for all of us and also Tremec.
Ed