Where are you from? I would consider a 750 with tracks. I don't recall if you can get tracks on a 670. I am sure you are aware that the 670 Lexion is a cylinder/walker machine. It definitely serves a purpose but most people prefer the cylinder/twin rotor hybrid.
With the header sizes you mention, I think you would be happy with tracks, and tracks on a Lexion allows much easier machine servicing, along with a great ride and load capacity. Turning radius is not as good with a track machine because the tracks prefer to go straight.
With those headers, you will need a heavy duty header drive. You can get constant or variable speed drive. I like the variable speed setup, but depending on the year, you need to verify it is the heavier version capable of those big loads.
Auto contour HP is what you want. That means it has lateral tilt. It should also have a dust suction fan on the feederhouse and a threaded top-link to manually and quickly tilt the header fore and aft. If it is new enough, it can have a hydraulic top-link for on the go header tilt fore/aft.
I can't stress enough, get tracks.
For 30-50 bushel wheat, there shouldn't be a lexion not capable of that material. Can't comment on milo.
A big consideration is a wide body or narrow body Lexion. Same machine but the threshing and separating is wider on the wide body. 670 walker machine, and 760 and above are wide body if I recall.