I needed a way to cut two perfectly aligned notches in the bottom of my frame to accept the rear crossmember. If I had the frame laying upside down where I could work on it easily I might have just cut them in with a plasma and a grinder, but there was no way I wanted to do that laying on my back under the truck. It took me some time, but I finally figured out how to do it, and do it accurately, without having hot slag in my armpit.
I clamped a length of 3/8" x .062 tubing from the outside edge of one frame rail, across the bottom of the frame, all the way over to the tube I set through the floor to represent the bottom leg of my main hoop. This allowed me to align the little tube front to back based on the main hoop location. Then I tack welded the tube to the bottom of the frame and cut it approximately flush with the sides of the frame. Now I had a guide for a 1.75" hole saw.