Jana, I'm not sure if this will help or not, but on the website www.findagrave.com there is a Benjamin McFarland (birth and death dates unknown) listed as being Sgt. Co G 5 Mo SM CAV.--no dates on the stone--the burial site is not in Polk County, but in Pine Lawn Cemetery in Houston, Texas County, Missouri. Texas County is not all that far southeast of Polk. However, I find no record on that site of a Cynthia McFarland (at least not in Missouri)...being a 3d wife, is it possible she remarried after Benjamin's death?
Also, on the same website, there is a Samuel Jesse McFarland buried at Rose Hill Cemetery, New Goshen, Virgo County, Indiana;
Birth: Feb. 17, 1846 Death: Feb. 6, 1917.
May not be the right ones, but possibly worth taking a look at...
Another possibility on the same website: Benjamin McFarlin (b. 1801; d. 1881) http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GSln=McFarlin&GSfn=Benjamin&GSbyrel=in&GSdyrel=in&GScntry=4&GSob=n&GRid=5944867&df=all&
Good luck in your search.
Maggie