</font><font color="blue" class="small">( Any idea what the proper rpm is for heavy disking or bushhogging for the 115 cu. in. E-350 engine? Assuming the engine has been broken in, of course. )</font>
There will be a lot of variation for both applications, depending on ground speed and conditions. You want to go slow or fast enough that you have control, and feel comfortable with the speed you are running. You do not want to put the tractor in too high a gear, so that it bogs down, yet at the same time want it in high enough gear so that you aren't revving the engine too high.
With disking, you want to go fast enough to break up the dirt, yet not so fast that you are leaving trenches.
With bush hogging your PTO speed doesn't have to be at 540, that is more of an upper limit. You do want your PTO to be running fast enough so that the brush or grass doesn't bog down the engine.
As you gain experience you will get a feel for what is right.