It can be done, but as others have already said, it'll be a tight fit. If you want the extra capacity for beans, go to a 52 if you plan to stay with the 4-38. Unless you want a 30' bean head, the 52 will be plenty big enough - 'specially if you Hyperize it. I wouldn't hesitate to run a 24' flex on our 52 for beans if we had flatter land, lateral tilt, and use the hose mod. Your 42 should be enough to handle a 20' head if you use the mods too. As a side note, there is a custom harvester near here that has run a 4-38 on a 62. He found it impossible to do an even halfway decent job - had to run too fast for the head to try to keep machine running near capacity - if he slowed down, combine would do a poor job. A 62 is almost too big for 6 rows when it comes right down to it in most years.