It's difficult to do it accurately with the tractor together and is best to do it by splitting. Having said that, it can be done through the hand hole. The problem is you need one hand for the wrench, one for the stubby screwdriver, and one for the measuring tool. It gets crowded with all 3 hands in there!
The levers should be adjusted so they are 1 1/4" from the plate and all exactly alike. I only did it once through the hand hole, and used a special and expensive tool to measure with... a very small stick with a drywall screw driven through it projecting exactly 1 1/4". I think a different tool would work as well... a stiff wire bent sharply to 90 degrees, and with one side trimmed to exact length.
The lever you are working on should be on the bottom and marked in some way so you are sure you get all 3. It would be easy to miss one when turning the engine.
Naturally, when you are finished you will want to adjust the pedal for free travel. On the '48, the pedal adjustment is on the rod on the inside, rather than on the pedal externally. You'll have to pull a pin on the yoke to adjust the length of the rod.