Yes, my frame has square threaded nuts welded to the inside of that mounting plate.

Because I'm working blind and from scratch it's difficult to tell from the pics on the tech tip and the Brother's site how the pieces in the conversion kit go together and on the frame. So with your indulgence I'll check with you on things as I progress, taking my best guess at each step. And maybe, at the end, we'll have pics to guide the next newbie through the process.
(if you'd rather do this through PM let me know)

Does the bracket mount to the frame as in this view ("view 1") http://i35.photobucket.com/albums/d188/pathwayrev/1959%20Chevy%20Pickup/bracketview1_zps51bdce43.jpg

Or like this ("view 2"):
http://i35.photobucket.com/albums/d188/pathwayrev/1959%20Chevy%20Pickup/bracketview2_zps2da0c366.jpg

(I'm guessing it's view 1)

A VW guy trying his hand at a '59 LB fs. Lots to learn, but it's about the journey.