Driveshaft: how much clearance?

About 3/4".
Did you measure it with the weight of the car on the rear suspension, or with it hanging?
Were you trying to install it with the weight on the suspension, or with it hanging?
You may not get it in with suspension compressed even with the correct length shaft- depending on the yokes used, it's gonna be real tight. Let the suspension drop and it should pop right in.