Thank you Prasanta!
I did find the “Interpolate Shape” tool, (and added the 3D length field) it DID give me a different number for 3D length, by 7 miles. That does not seem right (but ?) the 264 mile Reservation boundary mostly follows watershed divides in the Rockies with an overall 2000m (~3000 ft. to 9000ft.)elevation fluctuation. I’ve enclosed the profile graph it produced, which definitely appears to have worked (I’ve labeled physical features that appear to make sense – it runs from the NE corner, south to Evaro Hill, west to the Flathead River, north, then east back towards Flathead Lake and the NE point of beginning).
Thanks to all who have answered or responded, will run with this for now.