An M2 will handle a 18" quit nicely, in fact I think it's just about the ideal size. Is the head a Series 3/300 or is it an older series II with the Hart-Carter floating cutter bar? The older heads work OK so long as your on fairly level ground. The 3/300 series are much preferable as it is more "flexible" but usually costs more and is a little more expensive to rebuild.
The best thing about the older heads in soybeans is the ledge effect of the HC bar. Much easier to see and keep stones from getting into the machine! Good luck!
The best thing about the older heads in soybeans is the ledge effect of the HC bar. Much easier to see and keep stones from getting into the machine! Good luck!