Can i use 1965 mustang leaf springs on a 1984 truck?
Car springs are not made to carry the load required from a truck. You can buy helper leafs for your truck. Each one will add 1000 lbs to your carrying capacity.
Coincidently, I've owned both a '65 mustang (coupe w/289,200 hp, 3spd), and an '84 F-150 ( 300 six, 2wd, 4spd). I never messed with the suspension on the 'stang, but I did add 1 leaf to the rear axle of the truck. I paid a spring company, Jenson & Mitchell in Paterson, NJ $200 to make the conversion.