Jim you are definitely correct on the king pin twin I beams. Darth's were fully aligned in 1995 and other than some toe-in adjustments nothing since then.
If you can find a shop that can and will bend them, and knows how to set them up correctly, they will last forever, just replace the bushings periodically.
The fellow who did my 1977 and Darth, was training an apprentice when I took Darth to him, he asked if I minded having the apprentice do Darth's alignment under his supervision. I told him if he had confidence in him, then let him do the work so he could learn.
That was 27 years ago, still drives straight and amazes many people as to how even the tires wear. Shop I bought the tires from wanted to check things when they put the tires on, I told them ok, but other than toe-in not to change anything, he said they can't do anything on the king pin axles, he just wanted to see what it was set up at since the tires were so evenly worn. He came back to me and said it was the most perfectly set twin I-beam he had ever seen.
Bill AKA "LOBO"
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"Getting old is inevitable, growing up is optional"
Darth Vader 1986 F350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator
Wife's 2011 Flex Limited
Daily Driver 2009 Flex Limited with factory tow package
Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413