Why should the tractor be the old tractor when we are talking about a futuristic PV field? There is already a market for these autonomous tractors and seems they are all about half the height of a traditional tractor, basically the same vehicle with the cab cut off. Then for specifically mowers they can get pretty slender. I imagine a purpose built mower/hay harvester could be very slim. Huge advantage for the farmer if these are electric as well; power would be "free" from pv field and you only need a single technician or mechanic perhaps to maintain this operation.