FWIW, I HAVE a 6-354 Perkins from an "automotive" style install, clutch and SAE#3 clutch housing is still on it, if anyone is interested....... I swapped it out for a 4-71 Detroit in one of my boring machines.
I have toyed with the JD 4cyl in a pickup just to say I have done it and to see what kinda fuel mileage it would get, just haven't run across the donor engine, yet.
I am more familiar with industrial Deere than AG, so what is the size weight difference compared to a Cummins 5.9L in the engine of your choice? I dont see cooling even being an issue, since you will flow WAY more air to it than it is accustomed to being as it will be traveling faster than 25mph. Mounting and weight would be my first concern.
Since I do odd ball repowers at least once a year, I can tell you you need a GOOD machine shop. Look at his neat pic, you see that shiny ring in the clutch housing?
That is an SAE#2 to SAE#3 Clutch housing adapter with a 1/8" setback. In other words
I bolted a smaller transmission to a bigger engine and only set the transmission back 1/8".
That cost me a cool grand, to have custom built. There is one place in the country that makes them, but they were 3 months out on a production run and I needed it then.
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Good Luck.