I wouldn't do that. I'd go ahead and get a three gallon carboy. There is just way too much headspace in there, and I think it would oxidize. I understand that you'd purge the headspace, but that wouldn't be the way to go either. In wines/port, you don't want co2 in your liquid. So, you'd have to degas it (which would leave you all the more vulnerable to oxidation) and then you'd still have all that headspace.