First of all, the nüvi 3xx is not suitable to use the NTU version of maps. Use NT maps for your nüvi 350.
Second, there is no hack or FW update to remove the 2 GB map file size limit for the nüvi 3xx series devices. However... by using MapInstall you could create your own 2GB version of the European map and place it on an SD card. Then use MapInstall again to create another version of the European map, complimentary to the first one, and place it in the internal memory of the nüvi (as far as available storage space allows). This way you will possibly still not be able to use the full 3.36 GB fill size (2016.20 NT version), but at least cover more of Europe than the 2 GB limit alone would permit.