Draw an offset line 1/8" on the upright channel for the thickness of the bottom longeron. That line is lined up with the outboard trim line of the 6b5-2 rear frame channel.
Now I need the laser level.
Went to Home Depot and found one, a BDL310S for $24. Got it home and discovered the table was out a little bit, so I shimed it back to level and re-set the trial fit at the far end.