There is nothing wrong with the 6.2 or 6.5 if they are in good shape, and I would just stick with that. For one it's a factory setup and keeping in mind that this is a work vehicle that likely needs to be reliable and not let you down. I have no idea how big your farm is, but for me when I have multiple tractors/employees going and it's time to get across acres NOW, I don't have time for equipment letting me down, especially my support system which is my 2 Ford service trucks and my Dodge puller, that keeps the big iron in the field running. Last thing you need is something failing that got over looked doing a Cummins swap or what not when the push is on during the busy times. We all miss things, no matter how good of a mechanic we might be, it happens.
I went through something similar, I made a thread back in June about completely re-vamping my '81 K30 to make it more beneficial to my current farming practices, but I couldn't financially justify the cost of updating the power train, new bed, yada yada for a 35 year old truck. It was just cheaper to buy a newer truck, leave it factory stock and reliable, and is still at the end of the day a better, stronger, more reliable vehicle then a square body GM K30. (mid 90's F350 7.3 PSD)
I know we are all square body fans here, but for work, sometimes it's hard to justify relying on a 30 year old truck with 30 year old technology. But..... everyone's situation can vary.