CMB.... I would roughly think around 40-50 gallons. If you keep the tank the same depth as the main tank thats 12" the width of the main is 28" so the measurement you need is from the casting platform bulkhead to where ever you want it to end ...about 30". You could get fancy and have the bottom of the tank fit more to the shape of the hull but I would think thats bigger money and you probably would not gain a significant volume. Do you have anything under the console ???? There is tons of room in there to extend a tank.
SP Tanks is in Toms River NJ and they seem to do quite a business. there are several others down here if you dont like them. I know Otto purchased his tank through them and seems pleased. They also have a volume calculator on the site, just plug in numbers to get an approximate volume.....Or use L x W x H / 231