[GMC70c20]

I would say there's a trick or 2 left to do with the front end before you start working with rear end geometry. I'm assuming the front suspension is stock minus the shocks, it heavy plus throw a bbc and a turbo 400 at it and it gets really heavy. I would try reducing the front shocks to a 90/10 setting or something close then stuffing the lightest spring in the front end that will fit and not bind at ride height. This will put some pre-load in the front end so it will transfer weight easier. As for the rear the reason people say lower the front of the control arm is using "resultant vectors" the suspension will push in a line more direct to the center of gravity of the vehicle. Essentially what is happening is you are not transferring enough weight to the rear tires and the torque the engine is applying to them is more the the tractional force that the tires can create with the specified vertical load. You should try to get your hands on a copy of the book "door slammers" it has ALLOT of good info in it.