Any tips on fitting the US Car Tool Sub Frame connectors?

The only issue I had with the subframe connectors was about 16" from the rear they started to spread away from the rear floor pan and had about a 1/4" gap at the rear torque boxes. I just cut the connector at the 16" point, bent it up to hit the floor flush all the way back to the torque boxes. Then welded the cuts on both sides of the U-shape.

The other issues were rerouting the brake lines and re-routing the gas lines. Which you should upgrade to larger anyways.