1998 Jeep Cherokee overheating

My jeep received a new engine and transmission along with fly wheel and many other new parts. Once I got it back from the mechanic it was having black carbon soot coming out with a hard idle and overheating. Took it to another mechanic who replaced the air intake valve sensor and that solved the black soot and idle but was still over heating. Radiator and hoses are all new and I just bought a new temp gauge and put it in but it is still registering at 260 when its normal temp is at 210 (which I have read is normal for the year of my jeep due to the odd set up of the radiator & reserve reservoir). What could be the problem in why it still says its overheating?