I think you are unnecessarily complicating what to do to make a trike out of a Valkyrie.
Most trike kits lose the swing arm and rely upon the rear axle in the kit for the suspension.
The rear axles already come with spring mounting points and you can shorten a rear end in the proper manner to get the differential in line with the u-joint at the motor.
Connect the bike frame to the carriage in a solid manner and the carriage then becomes the top mounting point for the springs.
The best and easiest thing to do is purchase a kit. All the hard things needed are already fabricated and engineered.
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and works.