I clecoed the three #40 holes with long reach cleco's, clamped the spars together at the wing attach block ends with "C" clamps, up sized the three holes one at a time to #40, lightly reamed to a little under 1/4" and bolted them together. Then I drilled the attach blocks to 1/4". It worked very well since I did not trust myself to free hand the attach block drilling without egging the holes on the other side. This was in reference to the wing main spar plan, detail "K" which shows how to "mate" the spars and ream the holes out. Plans # SNX-W08, a note after spar assembly is bolted / riveted together, if that helps.
I also cheated a little before mating the spars and removed the two end wing assemblies (only three bolts each) from the spars, jigged them in the drill vise to keep them both the same, pre-drilled them 3/4's of the way through with a piloted 1/4" drill bit in the drill press, then re-installed them. After the three inner bolts were installed, I used the pre-drilled end attach blocks as my drill guide and followed it through the other block, flipped the spars over and repeated for the other end, worked great.