Joe, just change "big buck nigger" to "Railroad navvy" and you've got it.
"Screwing cotton" refers to using a cotton press to bale the cotton. The loose cotton was packed into a large box. A large wooden screw in a frame with a square plunger attached was then "screwed down" into the box to compress the cotton; kind of like a cider press on a grand scale.
This was repeated until no more cotton could be forced into the box. this made one bale weghting somewhere around 400/500 lb.
I have seen one of these presses in operation (it was mule powered) and it was a sight. The screw was hand-carved out of a pine tree trunk nearly three feet thick and 30 ft. long.
This was in "Westville" a restored 1850's town located in Lumpkin, Ga.
I always tailor my songs to my audience. When a chantey group I was once in were booked into an old folks home, we changed a line in "Coast of Peru" from "Where an man buys a whorehouse for a barrel of flour" to "a tavern". Afterward an old lady came up and informed us that her father had always sung "whorehouse" but it was probably best that we had changed it as "Most of these old so-and-so's are real prudes. I doubt if some of 'em have ever seen themselves naked".

troll