Combines generally use the GRAIN group, while things like balers use the SOLID group by default. If you made a group like I explained earlier for your windrow fillTypes, you could set all grain to use one unit and the new group to use another. You have to pick a volume AND a weight unit for each, so you can't just pick tons. The game is designed around volumes, but I added a toggle to show the mass/weight as well.
The easiest way to do what you want is to grab the UC addon and look at those xml files. Set everything up exactly like those (if you are using modSettings, otherwise just edit the addon). The thing you want to do is just remove the units you don't want to see and put tons before pounds on the list (or just leave pounds out).
It actually sounds more complicated than it is. If itch lets me, I'll show you a screen of how I think you want to see things.