I believe the Nuvi 760 supports the standard garmin protocol.
So you should be able to download your tracks in TopoFusion to a GPX file (from the eTrex).
Then, use TopoFusion to upload the tracks "as saved tracks" to the Nuvi this time.
Earlier versions of the Nuvi didn't support track uploading.
If you could get back to us on whether this works, we'd appreciated it. I don't currently own a Nuvi 760.
Thanks.