Bob,
There are two questions: 4 vs 6 speed transmission and the addition of a creeper to existing of either 4 or 6 speed.
The standard tractor is a 4 speed with 2 ranges - 8 speeds
The 6 speed also has 2 ranges - 12 speeds.
If you are just doing loader or chore work, a 4 speed should suffice
If you want more speeds for mowing, making hay or a particular speed for spraying at 4 MPH, likely the 6 speed is what you want.
Either can have the creeper added to add another 4 or 6 speeds in the bottom speeds. The tractor does not have to be split and it is readily done in about half a day by first timers.
Unless you are harvesting vegetables or blowing lots of snow, you likely do not want a creeper. If you are pulling high draft tools in primary tillage, you would not use the creeper here either.
Hopefully this will clear up any misconception on the M40 transmissions.
Good luck!