Yetter does have the manual, but it is not "exactly" the same as the JD manual.
http://www.yetterco.com/help/manuals/7012-150_planter_unit.pdf
The yetter manual shows planting distances based on mph, but the JD manual for the 71 does not. I have a JD manual. If you will PM me your fax number I will fax you a copy.
30 Row Spacing
Spacing / Drive Sprocket/ Shaft Sprocket/ Plate/ Plant population
7"/ 14/ 20/ 24 cell/ 29,900
10"/ 14/ 20/ 16 cell/ 20,900
With 60" row spacing you will get 1/2 of that.
Make sure you get the correct plate for your seed corn. Mine called for a B9-16. I had to search around to find it.
With the white field corn I bought I had to buy a B5-16 and B5-24x (same plate, but different # of cells).
www.lincolnagproducts had new plates, but there are several guys on ebay that have used plates. I also have an email of a guy with a bunch of used plates if you send me a PM.
Chris