1. If you are calling it a B7001 with a B7100 hood just because the serial number starts "B7001", be advised that is normal for an older B7100, it doesn't mean the hood has been changed.
2. Tire size depends on tire and rim type. Agricultural tires with a traditional "tractor tire" tread use different rims than turf fires.
Ag tires are specified by the rim size."6- 12" refers to a rim 6" wide and 12" diameter. They do not give any indication of the actual diameter of the tire itself.
Turf tires are specified by the tire dimensions, including the maximum diameter of the tire.
By the sounds of your description, I assume you have agricultural tires and rims?
3. Neither rim diameter nor tire diameter are the deciding factor for the 4wd ratio. What really matters is the "rolling circumference" of the tire, which takes into account how much the tire squishes under the tractor's weight. That's a number that manufacturers are reluctant to provide (probably because of the variability). The best way to verify it, since you have the wheels installed already, is to mark the tire and ground were they contact, move the tractor exactly one revolution, and measure the distance between the two contact points. Repeat for the other axle.
Now you can calculate the ratio, and compare to the spec for the tractor. If they are within a few percent of each other, you are good to go.
Edit: B7100 4wd ratio = 1.464:1