1969 big block Darts use a 7-blade 18" diameter 1-1/2" pitched fixed fan - #2863213
replace it with the 7-blade 18" diameter 2" pitched fixed fan - #2863224 (same as used on a Superbird to increase air flow)

replace your 160 thermostat with a 180 unit - once you reach 160 your thermostat is wide open and never slows the flow of water thru the radiator where it would have a chance to cool

do you have a shroud - do you use antifreeze - do you use water wetter? yes, yes, yes

where is your timing?