It's pretty simple to remove the old ones, sand the undercoating off. This will expose the spot welds.
Drill through each weld, doesn't have to destroy the whole weld, just get rid of "enough" of it.
Then, stick a prybar in between the rail and the car, and ream it down. It will pop any remaining weld, and rip your rail off. It will sound like a giant unzipping his pants. That giant is Yutaka Katayama, getting ready to f*ck you. Welcome to owning a 40 year old sports car.