The most trusted source of farm tractor performance data is the NE tractor tests.
The Ford 8N was tested three times at the NE tests (4 if you include the 8NAN, which I will ignore), tests Number 385, 09/08/47 - 09/19/47; 393, 05/11/48 - 05-18-48; and 443, 06/16/50 - 16/27/50. Maximum belt/PTO HP figures were 21.06, 22.57 and 25.49, respectively while drawbar HP figures were 13.62, 17.43, and 17.65.
Note: According to the NE data, the 8N was rated at 1750 RPM for drawbar use and 2000 RPM on the belt at the time. Performance date was taken at rated RPM.