I just put in a transmission in my 1978 Sedan Deville.it drives and goes in reverse. Even though the gear is in park the car still moves. brakes work fine when stopping. Its just that the car still moves when its in park.
how do i set the linkage? I tried several times. All gears work except park. Is my transmission garbage???

Sounds like the parking prawl is broken.

Chock the wheels, put the car in Park and remove the key. Go under the car and disconnect the linkage and manually put the linkage in the Park position. Now adjust the rod so it fits into the lever on the transmission.

If your car still rolls, you either have a broken pawl as Dan said or you have a bent park rod that goes from the internal linkage to the pawl.