....  a  Ford 300 on propane  'might' have to be spun at the higher RPM  just to make enough HP to handle the load of  4 submersibles, plus the pivot.    
Any idea of  the HP of each submersible?   if these are 15-20 HP pumps,  you may be  pushing the limit..
  a typical  7 tower pivot with endgun  could take about 20 HP  if all towers were rolling at the same time, and end-gun is running.  

let me ask you-   what kind of  fuel consumption are you seeing?    I have GM based  8.1's  pumping from 200 foot,  plus powering the  pivot,  burning about  8 gallons an hour  @1800 rpm.