I use a battery tender with an optima battery all the time when the car is not being driven. I have my drivers side window open completely to run the power cord out. In any event, I do not believe gasses from the battery are an issue. On the driver side I put the car cover over the driver side mirror. My '68 is not very air tight. I have been doing this for years now, and never had any problems with the battery which is about 7 years old.

Thanks guys for all of your help. For some reason I wasn't getting email notifications on your replies. Think I have that fixed now.