Subframe connectors
I cut out the whole floor and installed stamped steels rails the same dimensions as the rear frame rails. I think they were 3.75"x2.5" with 1" lips on the top to weld to the floor. Very labor intensive way to do it, but if you look at it from underneath it looks like it grew there from the factory. When I started the project us car tool hadn't released early a connectors yet. I would use their product if I had to do it all over again.