Thanks to everyone for advice. Memories of Timbo Horton's mishap, which appeared to be a broken swingarm from the photos, influenced the modification. Lots of metal was used in the extension and it is real strong. I do not need to add any ballast. The swingarm weighs enough.
After work this afternoon I put a wooden crate on the luggage rack, filled it with junk, put on my padded jacket, and took off down the Interstate. The freeway section was between Woodburn and Salem. Lots of trucks, truck ruts, fast traffic and a side wind. The bike has always been a handful on this road. With standard 117 mm trail (55 mm offset) it wiggled and hunted a bit due to wind turbulence and the ruts. The box on the back always made it worse. I could not ride the bike on this section with the flattrack triple clamps set at 113 mm trail (60 mm offset).
This afternoon the bike was stable. The triple clamps are set at 53 mm offset (119 mm trail). I rode along in the ruts, around all the trucks, through the sidewinds, no problem. One handed while picking my nose. The bike has the standard swingarm and the trail to wheelbase ratio is 8.0%. A happy day. Wobblywalrus lost his wobble.
Some late night calculations show that a set of clamps with a 48 mm offset will give me an 8% trail to wheelbase ratio with the extended swingarm. I will build my clamps using this offset.
There is nothing wrong with the 60 mm offset on the Harley flattrack triple clamps. The oval racers have much less rake than my Triumph and this large offset is OK with their fork geometry. If I knew then what I know now, I would not have used these clamps.
This is a humbling experience. Before this, in my dreams, I could see myself building a frame. If such small geometry changes give such large differences in handling, I will never get everything figured out when I build a frame. That job is best done by experts and the money they charge is well earned.