My TH350 was rebuilt and then sat for 7 years. It took a couple of days to get it back to where it would shift right. In fact, it took a few minutes at first to get the car to move when I first got it started. As long as it has fluid in it you won't hurt anything as far as I know. Have patience and put it in drive and wait for the clutches to engage. Then just take it easy on it and let it get used to doing what it's supposed to.