You are trying to put lipstick on a pig. You should have checked the vehicle specs before loading that much weight into your car.
My P5, with me in it, weights almost exactly 3000 pounds. Assuming you weigh around 150, and then adding in 1300 pounds more, your driven vehicle is 4300 pounds. Now look at the owner's manual page 10-6. The Gross Vehicle Weight Rating is at most 3691 (for the automatic), 600 pounds less than what yours is. Gross Axle Weight Rating for the rear axle on the P5 is only 1720 pounds, I'm estimating you have 2800 on yours.
If you want to make the car safe to drive, then you need to lighten it by at least 600 pounds, and most likely also redistribute the weight so that it isn't all in the back. Until then, the car is not going to be safe to drive (other than at 5 mph, onto and off of a trailer), no matter what tires you put on it.