I did it!! 325's on the rear of the 65 dart(barely)

I did my own sub frame connectors.......2X3" rectangular , and it comes up through the floor behind the drivers seat.
They butt up to the torsion bar cross member in front and enter the front of the rear subframe at the back.
Tying them into the floor really adds additional stiffness.
There are other ways and I seem to recall seeing kits for these cars too.
But it is a lot cheaper to do it your self.
I have seen round tube and 2X2 " that don't go through the floor , and there is a company that makes connectors that follow the contours of the floor pan with a "U" channel and weld to the bottom of the car that look pretty slick.
I can't recall who , but U.S. autobody rings a bell.