The 4 link (rear suspension) has to be considered to be a system. The 4 link brackets (axle and chassis) need to be in the car in the correct relationship to each other, in order to have the right geometry when it's finished and on the ground. Since the brackets are already installed, the installed height and horizontal location of the entire assembly needs to be determined and adjusted by desired ride height, rear axle location (wheelbase), tire size, etc. The final ride height of the whole car is limited by the stock front suspension. The front end of the car can only be made as low as the stock front suspension will allow. Bump stops and geometry only allow for so much lowering.

To get the car to sit level or even have a little rake, and obtain the correct 4 link/suspension geometry, final desired ride height needs to be known to get the rear clip installed at the correct height. Bottom line, the suspension geometry needs to be determined and located correctly and the car fitted to those locations. Imagine the rear axle located at the correct ride height for the tire size with the axle brackets installed, and the chassis brackets suspended in mid-air at the correct geometry and building the car to those brackets. Can it be made to work OK without going through all of this? Maybe, but you are losing a lot of the adjustability and advantages of using a 4 link if it's not right when you get done.

I have used pre-fabbed rear clips with the brackets installed with good results, but I always knew what the ride height was going to be, where the rear rails would attach to the rear of the body, tire size, etc, and had the rails bent and brackets located for my specific ride height and application. The back of the frame rails should be at about the same height on the rear panel as the stock rails were. Leave an inch or so of stock rail attached to the rear panel, finish welding the spot welded attachment, locate the new rails and weld them in using fabricated sheet metal gussets or bracing to join the new rails with the old to provide a strong assembly.