I would not use the key start unless you wired it in WITH the thumb start so you need both to start. Otherwise the thumb start is best.
from personal experience: I remember watching my Grandfather start his Farmall Super A (I have it now) by standing in front of the right rear tire and pulling the start lever to crank the tractor. I also remember watching him shake the shift lever back and forth to insure it was in neutral first. I got a Farmall 140 (very similar) and had pulled the shift cover and moved the gears around to diagnose a problem. I reinstalled the shift cover and had to slide the gears around to get it back on. When I went to start it, I stood in front of the right rear tire, shook the shift lever to make sure it was in neutral, then just before I pulled the start lever, I had a moment of concern and stepped back from in front of the rear tire and pulled the start lever. The tractor cranked and I then had to run after it and climb on it from the back to shut it off since I had inadvertently transposed one of the gears. Neutral was effectively 2nd gear! I am able to walk around and talk about this because I got lucky and moved out from in front of the rear tire. There was no way I would have been able to react to the tractor starting in gear and it would have run me over. I now sit in the seat of my Super A when I want to start it, every time! My 8Ns both have the thumb starts. One of them I replaced the same week of that incident.
As a tractor newbie I started out with a big bag full of luck and an empty bag of experience. I moved some luck into experience that day. I just hope hope my bag of luck holds out until I fill my bag of experience.
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