put back in correct new 195d F thermostat....fill with 50% antifreeze mix....park so front end 1>2 ft. higher than rear or on ramps or stands....leave radiator cap off....run engine until it reachs operating temp with heater on max then off several times...top off coolant...put on new radiator cap and turn to first click, about 1/4 turn....run engine for 20 minutes varing rpm up to 3k rpm with heater on max and then off....
FoMoCo shop technique to remove cooling system air

if this does not work, then you have the infamous leaking 3.8L head gaskets with yours leaking hot combusted gases into cooling system...