20 feet is way too much header for this combine. A rule of thumb we used to use here in Ontario was 15'foot for a 1420, 16.5' or 17.5' in a 1440, 17.5' or 20' in a 1460, and 20' or 22.5' or 25' in a 1480. Bear in mind that the 1420 and 1620 were low production machines and are somewhat orphaned for parts in that the non common parts are starting to be hard to get. I would suggest a 1460 or at least a 1440.