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

use floor jack to hold them in place, i also ground down some places to better fit the floor contours. also bevel the edge you are welding so its closer to the floor pan metal thickness, will make it easier to weld.

i drilled extra holes for welding surface area were it slips over the rear frame stub. they make a great product, you will be happy you installed them,