Subframe connectors

It seems that if your car is sagging in the middle and you suspend the car "in it natural stance" IE on the wheels, you are locking in the sag. This may be ok if all the doors line up etc etc.
But if not you may want to somehow also support the car in the middle of the car when attaching the frame rails.

Or your car could be bowing in the middle if you have a very heavy engine, in that case you may need to support the K-frame.

I guess it all depends on your car.