IT is BS that a Titan II will shake itself a part. You just need to replace the shoe/chaffer frame rubber bushings. Guys never check them out and then start to break the chaffer or sieve and then blame the machine.
They speed the shoe and chaffers up. They also have the dual range cylinder that the early one does not have. The elevator adjustments are improved on the T2 machines.
If you can pickup a Titan II for $6K more than a 1979 then do it. There are a boat load of improvements to the Titan IIs.
Also down the road that 1979 model 7720 will sell for scrap price verses the Titan II. You can buy JD early models for less than 10K all day long.
I would find it hard to believe the hours on a 1979 machine at being only 3100 too.