To the best of my knowledge, there's no way to do that with a MegaExo Gossamer. The way MegaExo works would require the crank's spindle to be 5mm longer, because the left arm has to bottom on the end of the splines, and on a 73mm setup they'd be recessed into the non-driveside bearing. Looking at FSA's documentation, I see no options.

One workaround is to pay to have the frame's BB shell milled down to 68mm width.