Brad, the 3" section is vertical.

The rear frame rails were opened at the end and had 5x9/16" or so holes drilled in each side (for a total of 20 holes).
The SF connectors were slid inside the frame rails, welded at the open end, then each of the 20 holes were also welded to close them and attach the connector to the frame rail on three sides.
There is nothing to attach them to at the top side, which again helps make the case for running them through the floor.

At the front, the connector is first welded to a flat plate which is then welded to the trans crossmember.

For a 69 B body you need two 4 foot (even) sections of .083 wall 2x43 tubing to make them, and the connectors themselves don't need any more work to fit, other than welding a plate to the front.
Rich