spark plugs

I'm having trouble with my duster overheating. My question is can different heat range spark plugs significantly alter your engines operating temperature. Havn't really changed anything other than running hotter plugs. 400, mild cam,750 holley,4.10 gears, automatic 727 in a 72 duster. reached 225 degrees when temp was close to 90 degrees outside. Not a problem in winter.