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Why don't you have a frame shop take a look at it before you start making changes. My first 71 needed the frame fixed first and that made some issues go away. Otherwise you may be chasing your tail making the changes based on a tweeked frame.
I may have slain the dragon here. I did use an engine hoist and a 3 pound wrench and was able to loosen the claps and cussed a little.
I have an equal gap across the back. Upon closer inspection the halo line was not straight.
So the fix was to lift the corner with the hoist and beat on it a little, which moved it forward about 1/4"; adjust all the pins & clamps; do a bit of mud therapy to the rear gap and sand straight.
Thanx for all the helpful inputs
Sam