its hard for me to believe that an FMX could be damaged by a 390 unless its putting out well over 1000HP...... fmx's are indestructable.... my buddy has a pantera running a 351C and an FMX, car is dynoed in at 700hp and hes put about 45k on it with that fmx and it hasn't missed a turn......

the gearing would change your gas mileage either way depending on what size tires you're running.... if you running 35" tires then a 4.90 gear would bring you gas mileage back up, but if you're running say... 185/70/13 then you would want to run like a 2.73 gear to get your gas mileage up.... pretty standard tire for a 65 galaxie would be like a 235/70/15.... which you would want about a 3.25-3.50 gear for optimal gas mileage