You will find out that the sub frame connectors, will in most cases, be too far away from the rear cross frame to be installed first... The pic below
IS from the US Car Tool website... In the picture he posted, the sub frame connectors won't land that close to the rear factory cross frame. there will be a gap. That gap can be as much as + 1/4".
The OP was asking for the ORDER to weld them in. I've done it 3 times now. The torque boxes were designed to go on first.
View attachment 1715184717