"North Canadian River at Eufaula", as you said, would be in the vicinity of North Fork Town -- which was located north-northeast of the present town of Eufaula, under Lake Eufalua, and nearly directly east of the Eufaula Hospital on Hwy 69. The boundaries of the Creek, Cherokee, and Choctaw Nations come together at the mouth of the North Fork of the Canadian (i.e., North Canadian) which might explain Young's belief that the free Blacks and slaves were in the Cherokee Nation, though in my opinion his information was probably convoluted as is typical of rumor and hearsay. This may have come from 'movers' (i.e., travellers just passing through) who didn't know the exact boundaries.