PRS (
http://www.precisionracingsystems.com/products/26) makes 64 pitch spurs down to 72 that should work on a TC3. I've personally used as low as an 80 PRS spur in 64-pitch on my TC3.
Robinson Racing (
http://www.robinsonracing.com/catalog/64pitchspur.html) makes 64 pitch spurs down to 78 that will work on a TC3. Robinson gears can be purchased on Hobbytalk.com
If you are using 64 pitch gears, you should be able to run as tall as a 46 tooth pinion before having to modify the chassis under the gear. I can't recall if it was a 47 or 48 that started to rub the tub chassis.
The only issue with the TC3 or TC4 is if you use a very small spur, you may have problems moving the cam over enough to make gear mesh with a smaller pinion. I would think a 74/44 combo should work. Looks like you are trying to gear to 4.20 FDR on the TC4. You may want to purchase a few larger spurs to fine tune your FDR as many of the fast racers that I've seen are closer to 4.4-4.5 than 4.2.