How do I fix my 240 GB upgraded Ipod Classic?

I recently bought a 7th generation Ipod Classic that was upgraded from 160 GB to 240 GB. I did a restore on it, because it wasn't working properly, and now it will only let me sync as if the ipod is 160 GB. Apple can't help me because it was custom built. The biggest annoyance is itunes indicates there over 70 GB 'free', but won't let me add any more songs to the device. It also won't tell me it's full, so I have no idea what songs it's actually going to sync. I'm not a tech person and could use some help. I only bought this ipod for the storage and am pretty bummed I lost it after only playing it for a few hours.