Well I got finished installing my lt1 into my car and now it rolls when in park, reverse, drive, 2, 1, when the car is off and on a slight hill. This is the trans that I had in the car before the swap and never had this problem. The only thing I can think of is the trans is a BOP and the lt1 is obviously chevy and I had to use a tranny adapter and maybe it seperated the trans/engine too much and the converter isn't on right. It drives fine and shifts hard but just doesn't sit still when off.

 

Make sure your shift cable is adjusted properly.
It could feel like it goes all the way into park but is just out enough not to engage the park mechanism.
It may even click as it rolls, I have dealt with this problem on several different cars over the years. It may not be your problem but something to check.

 

Sorry, I forgot to add that I disconnected the cable and shifted it by hand on the tranny itself and it did the same thing. It also rolled when I did the same thing and left it in gear. I'm a little confused lol!
EDIT: I checked if I put the flexplate on backwards today and it's fine. I forgot that the tranny adapter came with some washers to put between the flexplate and converter and I DID install them so now I'm even more lost.

 

That is odd.
Now I am thinking its internal.
Might pull the pan and check around in there.

 

Well I went under the car and I tried to put it into park again with my hand. It didn't go any further then it was at...then I put a pipe on the bracket and it went further. It was really tight but should go that far with the cable. The cable isn't adjustable so what the heck can I do? Try to bend the bracket foward a little?