Support it with blocks under the springs where the stud is that holds the spring pack together.
Then move the diff and perches it to where you want it and tack the perches on.
That's what I did and it worked fine.
As far as the car being level, just go off your trans pan rail or output shaft housing end and subtract your preferred pinion angle off of that.
You could turn the car on it's side and still know where the perches go for angle.:D
