Major transmission fluid loss!
 

I recently aquired a 72 GMC 2500 custom. I noticed that all the transmission fluid would leak out if the vehicle sat with out running. I think it is the seal around the transmission pan, but would like a second opinion. Any help would be greatly appreciated.

Thanks

I would clean it up with engine cleaner then just get under it and look around because it could be a many things since it's so old.