I am a first timer doing my ls swap, but I am putting in a 4L80E so it is an inch or so longer than the stock 4l60. So, I put my engine about an inch forward so I dont need a custom driveshaft. I measured from the motor mount to the trans on the original and measured the back of my block to the motor mount on the ls. That way you can figure out where you need to mount it based on the length difference of the transmission.