You can do engine and gearing mods and your bike will do 75-80mph much better. I am sure that my bike, Verslagen's, old_indian and Superthumpers bikes will all do 80 without problem (I don't even use 5th until I am over 60mph).
You need to a Wiseco, a cam, do some head porting, and change the gearing (Kawasaki pulleys if you want to keep the belt - or a chain and sprockets), a 140/90-15 rear tire. You are most likely going to spend $ 1,000 - $ 1,500 getting it all done - but the results are very good.
Or.....maybe you just need to look around a bit. There are lots of bike models out there that I have overlooked. A parallel twin will most likely be the best bike for getting down the highway at speed - my little Ninja 250 can do 75-80 all day long...and the parallel twin bikes tend to be smaller and lighter than the V-Twin bikes.
I didn't realize that the Kawasaki Vulcan S had an earlier bike that is very similar, and can probably be found for a lot less money. Some of the early models were belt drive, later a chain.
http://www.ebay.com/itm/Kawasaki-Vulcan-500-LTD-/142300285159?hash=item2121c2... https://www.cycletrader.com/search-results?sort=featured%3Aasc&radius=150&lay...;
The Suzuki SV650 is supposed to be a wonderful bike.......the early ones (pre-Gladius) are better in some ways than the later bikes.