I've built two stitch and glue boats in my poorly ventilated basement (right near the furnace). I'm also a chemical and environmental engineer that is reasonably familiar with health and safety issues.

I'm not sure how the density of epoxy fumes compares to air. My experience was that I could only detect a very slight odor when working with epoxy. I'm sure that I was exposed to nowhere near an unsafe concentration. Personally, I am not allergic epoxy fumes. A small number of people are, and need to take far greater precautions to minimize exposure.

As for fire safety, the lower explosive limit for most organic vapors is in the low percent range. I would guess that the actual concentration in my basement was in the low part part per million range. I can't see how you would get enough vapor in the air to cause a fire hazard, because the resin contains no solvent and the vapor pressure of the resin is so low.

On the other hand, use lots of ventilation if you solvent wipe the boat. That could be a serious exposure problem as well as a fire hazard. Be careful with solvent soaked rags, too. They can spontaneously combust if improperly disposed of.

I think the biggest safety hazard is related to controlling sanding dust. That stuff can be a real irritant. Definitely wear a mask and vacuum up the dust as needed.

Just my two cents worth. I hope it helps.

Dave