I imagine there'd be 20 point or 30 point deficits in favor of Democrats instead of 50+.
Seems about right. The only real differences in elections between 1880 and 1932 would be that the Deep South would have been a swing region (that would have voted Republican in 1888, 1896, 1900, 1904, 1908, 1920, and 1928) and that the Republicans would have won the popular vote by decent margins in all elections except 1912.
Actually, given how much The South loved Bryan's populism initially, and given how sympathetic The Populist Party was to black people initially...I'm pretty sure the margins would favor Bryan, even if only by about the same that Carter won in 1976.