I did a very similar trip back in '03. I had an XR650L and my buddy rode a KLR. Made it from Nashville, TN to the Panama Canal. Ended up shipping our bikes back from San Jose, Costa Rica (that due to my buddy having a nice spill in the highlands)
Tierra del Fuego was the original goal but the trip ended up costing us a bit more than we initially anticipated.
PM me if you ever need suggestions, tips, things to avoid, etc.
Good luck. Nice bike