Well here goes my idea- Mount a jack shaft directly below your pto set between your axles so as to line up 2 pulleys sized to a ratio of 3-1. The saft will have on the business end 1 3/8" standard slpine. A spreader like mine ( now I'm mostly sure about this)
you can turn the paddles around so it will throw in the opposite direction as you'll be turing the spreader gearbox the opposite direction. Without something gear driven you can't change the pto rotation so you'll be stuck with counter rotating pto. My spreader gearbox (now I'm mostly sure about this)
can go either way. Now, if all that sound to messy you could mount an electric motor driven spreader and even if your tractor isn't 12V a spare batt would probably get the job done. It wouldn't be to hard to hard mount something like that behind. I considered that option before what I did. With all this said, what drove me to the set-up you saw pictured was after about 2 seasons of pushing a broadcast spreader I decided there had to be a better way. I can load enough fertilizer in that unit to do easily 5acres or more without reloading. Takes longer to mount and fill than to spead. Worked so well that I got my neihbors together and I do everyones yard at once. Also If you don't already know this, Core type plug first and you'll get 98% yield on your seed.