I've heard of people running 6-row 30" on them before but it would be all it wanted. I think if you had very high yields at all you would be crawling not to overlaod the cleaning system. Not to mention lots of weight on the front of that machine. I've heard nothing but praise for the little 1420s as long as they are used within their boundaries. I have a 943 for my 1420.
Yes, the later ones (125 hp) were rated for 6 row. Open the bottom sives wide open. If you plug the clean grain elevator, swap the drive pulleys top to bottom, this gets you 15% more speed. It will also help to give the fuel screw a 1/4 turn, some 1420's came out very weak. TORQUE