Try 35 shore when its cool (65 degrees or less), 37 shore when its a little warmer (65 degrees - 80 degrees) and (40 shore for 80 degrees to 95 degrees) - lastly, if, it's extremely hot or the track has a ton of traction, try 42 shore. You should be able to use 26mm front and 30mm rear without a problem. Nitro cars are designed to use those widths. Give either Ellegi or Power Tyres foams (basically the same tire made by GP) a try. Also, give, my current and Stateside nitro tire favorite, Twistertires a shot. They're really good!
Good Luck.
