First off, I have read myself into utter confusion over bottom brackets/cranksets...so I am looking for some help before I spend unnecessary cash.
I am currently stripping parts from one "cross" bike to a new frame. The current bike has an FSA Gossamer/MegaExo 68mm crank and bb. The new frame has a 73mm shell.
Can I purchase spacers to adapt the bb to the wider shell? Or do I need a completely new crank/bb setup? If so, entry-level ideas for a double crank set-up? All the info on 73mm shells appears to be for MTB applications.
Thank you for any help you can offer.

You would need a BB for a 73mm BB shell. Your LBS should be able to help you out. Even if you have to take the frame to them. That way they can a perfect fit, most LBS's have a good selection of BB lengths and widths.

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.

Thanks, that might be what I do. Unfortunately, I don't have a huge amount of confidence in the LBS closest to me beyond simple adjustments. Plus, I want to learn how it's done correctly as part of the entire build process.
Regardless, I appreciate your response.

I might just look at buying a new crank/ bb set-up if I can find an appropriately sized combo.