Any proposed site within 320 km of the border must comply with international interference rules.
There is a separate spacing table: See 73.207(b)(2) http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=4c9e24a82511b548400667812a20e5de&rgn=div8&view=text&node=47:18.104.22.168.22.214.171.124&idno=47
Under the Canada-United States FM Broadcasting Agreement, domestic U.S. allotments and assignments within 320 kilometers (199 miles) of the common border must be separated from Canadian allotments and assignments by not less than the distances given in Table B, which follows. When applying Table B, U.S. Class C2 allotments and assignments are considered to be Class B; also, U.S. Class C3 allotments and assignments and U.S. Class A assignments operating with more than 3 kW ERP and 100 meters antenna HAAT (or equivalent lower ERP and higher antenna HAAT based on a class contour distance of 24 km) are considered to be Class B1.
Also, in overlap situations there are different contours that are used. Instead of 100, 60, 54 & 40 dBu, what matters is 100, 60, 48 and 34 dBu.
“I think the agreements between Industry-Canada and the FCC allows both countries stations to interfere each other.”
Not really. It depends on whether it’s interference caused or received (meaning which contours are offending).
Also, you can’t (repeat CAN NOT) examine international relationships without accounting for reserved frequencies and certain vacancies that must be respected as if they were fully operating full power within their class stations. In addition, the international rules have changed, and those stations already operating are grandfathered in even if they don’t meet the new restrictions (but no complaints are filed), but when you make a change of location or power/class, you are subject to the latest version of the rules.
If I can find my notes about Canadian/US interference I’ll add later what I am not remembering, but suffice it to say it’s a lot more complicated than US/US compliance. A lot more complicated. Mexico is worse.