The best and most robust dolly I have found to date is the BikePal, made in Canada.
http://www.thebikepal.com
It is claimed to be good for up to 2000lbs and is certainly long enough (84") for an F6B. But.....it's not cheap.
When I emailed them for more information and a price (something they can't seem to be bothered to put on their website... always a huge warning sign), I received what appeared to be a mostly-boilerplated response. Most interesting in it was that '...BikePal retails for $695 CDN or $545 USD plus shipping....'. I responded with the fact that the current rate of exchange made 695CDN equivalent to only slightly over $518USD and did they still intend to charge American customers the higher amount?
....as yet, they have not bothered to reply. I can only imagine what they wanted to soak me for shipping. No doubt that would have been padded in similar fashion.
The search continues....