Sourced the longerons at a specialty timber yard that supplies musical instrument timbers. While considering hoop pine I went for the expense and very high quality sitka spruce. I choose a piece with a 45 degree grain rather than quarter sawn to give stability in both directions. This is also how its pictured on the plans. Grain direction is not mentioned.

The pic shows the sized pieces taped to the excess timber. One piece still needs trimming to a triangular shape. The others were cut to the plans specs.

Longeron timber – Sitka Spruce