>I don't know if Bunyan pronounced it slu or sluff.

I don't know how Bunyan pronounced it either (or either), seeing as hough how Bunyan is still dead; but the OED pronounces it slow (rhymes with bough), sloo only as a dialect form, and refers to slew as a US or Canada spelling/pronunciation.

They drew near to a very Miry Slough... The name of the Slow was Dispond. - Bunyan (1678) [OED2]

NB: regarding the variable spellings in Bunyan - link

-joe (flat foot floogie with a floy-floy) friday