Yup. Car should have its normal stance when you weld these things to the frame. I think I paid 100 bucks for my install (coated MMs). They welded them up and then painted over all the welds. Really helped the vert feel a bit less cheaply built. Far less creaking etc.
You don't have to be on the wheels, but the suspension has to be loaded. You can have the car on stands, as long as the weight is still transmitted through the chassis in the same manner as it would be with the wheels on the ground.