You have to be extremely careful doing this.  Yes, it can be done in a pinch however it is quite a bit of torque on the upper.

You won't hurt the block, but if you are not careful you can hurt a receiver.  We fully recommend you torque flash hiders, compensators, suppressors, and other items with this kit.

You can also use it to hold the upper for mounting scopes, and AR accessories quite nicely!