Hi David,
At a contest, long long ago, I asked Terry why his Winder's wings never failed, while some others had wing failures. He said: "...... I use spruce spars, not balsa ..... ".
I put spruce spars in mine and never had any problems. It was a great flying plane.
If you are going to use a competitive engine, you may want to consider spruce spars (or balsa with CF). If you keep it under 100 MPH and don't intend to drive it to the max Gs, then balsa should work fine.
PS: One day Terry brought one out set up for RC with ailerons. He had a .35 combat engine on it. When it was launched it was almost uncontrollable, and at a very high speed it was close to out of sight in seconds. Terry's next RC Winder had a .15 on it, with much less AL travel, it was a fun airplane. He was way ahead of his time, now some 40+ years later, I think it would be competitive in today's RC combat.