It sounds like the driver of value is more the amazing period detail rather than size, since there are numerous 2800-3200 sf regular houses to be found. Typical buyers for period detail are a niche market. Look for other detail/period house sales or listings in region and see what they sold for/market exposure etc.
These kinds of assignments one has to play with them a bit...look up land sales, look at period detail houses that sold, talk to agents, etc. Don't be rushed and ask for time extension if need it. After you poke around a bit, develop a working location adjustment ( you can always change it ), find some period homes that sold, put them on grid etc, a value starts to emerge.