You don't need a 1680 to run 3-4 mph in that kind of beans, nor do you need that size machine to push a 6-30 cornhead. Brad Bish, Harv's Farm Supply, Giltner, NE is the guy to talk to about adapting a Gleaner head to a CIH combine.
I had a 1660 and ran a 25 foot 1020 platform and 6 row cornhead. Speed in beans was usually between 2.8 and 3.4 mph depending on yield. I upgraded to the 25 foot head as I was having to drive faster than I could cut with a 20 foot head. bryon
I just traded my Gleaner M2 for a 1680 with a 25 ft flex and a 6 row corn.I wanted the added hp,larger bin and larger feeder housing.There are others in my area that run 1680s with 6 row heads without a problem and I have a combine that will run and 8 row if I ever want one.You will want a specialty rotor if you are doing many soybeans and field tracker if you ground isn't flat,a friend of mine tried putting a Gleaner A 630 on a 1460 and said that he couldn't get the ears to feed into the combine right and gave up and bought a CaseIH head.I don't think Bish lists an adapter for that setup.
I used to run a 1660 (190hp) w_20ft 1020 and 6RN 1063. It was a high houred machine and I wanted to run quicker in beans (a 25ft head is a little to large for a 1660 especially w_green stem beans). Now I have a 1680 (91, cummins@235hp, long chaffer system, old fan) and it's really nice to have lots of capacity. I usually go 5mph or better in corn (still 6 row) and around 4 to 5 in beans depending how they are cutting. It's really been a nice setup and the 1680 was cheaper and fewer hours than the 1660. I wish I'd bought the 1680 from the beginning. Pat
Field Tracker was offered on 1992 combines,but there are quite a older ones with it as CaseIH offered it as a kit.I don't think it will add that much to the price of a used combine and a large number of later 25ft 1020 heads are Field Tracker Equiped.Go to www.toytractorshow.com_caseinternational_axial-flow_combines.htm You will find history of combines and when they were updated.