I finally decided to tackle the cracked subframe on my 73 swinger. I cut out the bad metal on the bottom where the rear kmember nut is and removed the nut. I ended up internally sleeving it with a piece of 3 by 2 inch 3/16th wall steel tubing. I did 3 3/8ths plug welds on the inner side and 5 on the outer side. Drilled the ends of the cracks and beveled them with a carbide bit in my die grinder. Then welded it all up and ground it smooth. This seemed more daunting than it actually is. If anyone ever needs to do this, I took some pictures that may help show what I did. Make sure to measure 500 times and cut once!