where to find data on outside axle widths?

the TractorData site lists weight and wheelbase. Is there another site that would also list outside axle width?

Lots of tractors have different length axle lengths available.100" are the long ones.

For example a 1066 may have either 100" or 118" axles.

I think the answers given so far aren't quite addressing your question as I am reading it. The width numbers usually listed for tractors are the minimum/maximum tread widths. These are the measurements from the center of one track to the center of the other. The overall width of the tractor would often be the maximum tread plus the width of one tire. I believe you are asking about the Actual distance between the tips of the axles. Some might refer to that as the minimum shipping width. If that is what you are asking about:

I don't know of any single source for that data. I have it for a few tractors. For some tractors, the maximum tread and the axle width are pretty close. For example, the Farmall C has an axle width of 80-1/2. That provides for a maximum tread of 80 with an overall width of 89-3/4 (depends on tire size). Other tractors have wheel centers with a larger offset and the axle width may not be that close to the maximum tread. For example, the Farmall H has an axle width or 75-1/2, maximum tread 80 and overall width 90-3/4. A Farmall M has the corresponding numbers 88, 100 and 84-1/2. The HV and MV have axle widths of 85-7/8. These numbers are all for standard axles.