I used walnut for the drag / anti drag wire blocks. This wood is harder than maple. The grain in the blocks should be horizontal [run from inboard to outboard]. This will result in less chance of the blocks splitting under load from the wires.
The plans call for the rear blocks to be 2 half's paired. I decided to make these blocks one-piece, and I cut a 1/4 x 1/4 dado in the center so the blocks would sit flush over the 1/4 ply protrusion at the aft of the rear spa [similar to the upper wing blocks]. As I now understand it, either the drag, or anti drag wires can be under load, but not both, so if made in one piece, any load from a drag wire can then be also transfered to the other half of the block.
As portrayed in the pic top right, I had to tack the blocks together in order to rip a dado through them. Why didn't I cut the dado while the lumber was in one larger piece? Well, the lumber I had was in short pieces like 3" x 12", and the grain went with the length, so this would have resulted in vertical grain blocks instead of horizontal grain.
In the plan pic below, ref' the comment "so what does this 3/8 refer to?" Actually refers to some confusion I had regarding spacing. The space is 3/8, yet the ribs are 1/4.........this was cleared up by a guy on the Biplane Hangar support website - the trussed type ribs would have been 3/8 due to the 1/16 gussets each side [1/4 + 1/16 + 1/16]. I am using routered ribs, so no gussets, hence the 1/4 inch spacing.