After measuring and marking for slotting and trimming the longerons I test fit the left one before making any cuts just to verify the dimensions on the drawing. I figure a longeron would be difficult enough and expensive enough to replace to warrant the extra care. On first fitting it looked like the aft end was going to come out about 1/8 inch short. After much study, I decided to erase the markings and remeasure. In doing that, the results came out exactly the same, so I went ahead and cut the slot and refit the longeron.
It must have been parallax making the aft end seem short, because with the longeron horizontal flange nicely nested around the HS attach bar, the length came out perfect, leaving about 9/32 edge distance for the aft most skin rivet hole through the vertical flange.
With that concern taken care of, I slotted the right longeron and trimmed both per plan. After fitting both longerons, it appears as though the aft end of the horizontal flange of the right one may need to be trimmed a bit more (photo2).