Being an Engineer I always look at the applied load vs how the designer intended the product to be used. The reason a spreader bar is used is to keep the load on the dinghy eye pads as designed, directly upward.
The chain usage in the post above applies a horizontal load (inward) on the dinghy. It may be fine but fiberglass does not give you any warning before failure, it cracks.
The spreader bar needs to have a thick vertical cross section, like a 2 x 4.
Typically SS or Aluminum tubing is used with a welded stiffener on top. 1/2" x 4" is usually strong enough. Tube diameter is a better characteristic than a thicker wall.
Tom and Cheryl
1982 41' President