as said before check and be sure you have a vented gas cap.
auto gas caps are not vented, someone may have installed a auto gas cap
my to35 has a metal shield under gas tank and its over the exhaust manifold. check with someone else who owns a tractor like yours that is not having the heat problem, see if your tractor has the same heat shielding as the tractor that does not have the heat problem.
maybe someone on forum will make a picture of their tractors heat shield.
need to get this problem fix as this could be a very serious safety problem.