I'm guessing your "Ford" manual is actually an I&T manual. My Ford factory shop manual (covers 600 series up through four cylinder 4000) has the same adjustment procedure that Royse showed, except it says to start "1-1-1/4" turns out rather than than 1-1/8. (I assume they meant to say "1 to 1-1/4 turns".)
Pulling three plug wires to set the main jet seems pretty goofy, but is it really all that preposterous? I don't think so. First, the only right way to adjust the main jet would be under dynamometer load. But not many farmers own a dyno. So you want to simulate heavy load conditions: Low manifold vacuum (aka high manifold air pressure), high carburetor airflow, and high engine torque. If you pull three plug wires and open the throttle all the way, you'll have low manifold vacuum and reasonably high airflow through the carb. Even though the three dead cylinders aren't putting out any power, they're still pumping air. The engine isn't putting out any torque, but the dead cylinders do present some load to the single live cylinder. I think it sounds like a reasonable way to set the main jet without a dyno or similar PTO load.
As for setting the main jet to eliminate "stumble" on acceleration, unless a carburetor has an accelerator pump or is a constant velocity carb, there's always going to be a slight stumble when you suddenly open the throttle. If you try to eliminate it, you'll probably end up with a rich mixture.
On the other hand, if you try to adjust the main jet at high speed but no load, the idle circuit will probably still be in play, enrichening the mixture. You're likely to end up with a too-lean setting when the engine is under load.
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