Subframe Connectors Install

It kind of depends on a few things. Does your car sit good and everything lines up well with no binding of your doors when you open or close them. Is all other sheet metal in good shape? (floor boards, rockers and frame rails)? If all is good then you want to weld them in as the car sits because it will stiffen up your frame quite a bit. So for instance if your doors bind slightly when you jack the car up off the ground and then you weld those connectors in while jacked up then you'll be doing some re-aligning. If you need to get it up off the ground to get under it to install your connectors then you could use jack stands but the need to be at the four wheels to allow the car to sit as it would on the ground. So the front should be supported under the lower control arms to put pressure on the suspension just like on the ground.

Now if you need to correct some frame sagging, that's a different issue.