Does the fan on an older 1970 or so wesco electric furnace start on slow speed or directly on high speed? Having trouble with the unit cycling off too soon when first starting up in the am. Have replaced the sequencer relay and have the fan off switch set as low as possible without the fan failing to shut off at all and blowing cool air. Around 80 degrees.

A 1970 unit most likely only has one speed. Is this a belt driven fan?

Your problem is likely due to cold air being drawn through the furnace & shutting the fan off due to low temperature. I've never seen an electric furnace with a temperature controlled fan.