When a car is overheating, turning on the heater will cool it a little, because the heater fan blows across the heater core and reduces the heat some. Even though you may not need a heater, not having a heater core can elevate the engine temp. Even when the heater is off, the heater core helps cool the engine.
I'm not sure a 68 had a fan clutch, it may be a direct drive, same speed all the time. My 67 was setup that way. It might be the wrong size fan blade.
chevy runs deep, Don't step in chevy!
67 Coupe 289 - TCI C4 w/2k stall; Holley 4150 570cfm; Performer intake; mild cam; Pertronix HEI; 302 heads; Roller Tips; Hedman Tri-Y headers; H-pipe w/Flowmaster 40's; 9 in posi 3.50; 205/50-17F; 225/50-17R.