I don't think that particular sight glass will work effectivly for this application. If your tractor is like mine where you fill to the bottom of the plug, you'll see little if anything in that style of sight glass. If you could find a piece that threaded in place of the plug you'd be better off.
Check out the link to McMaster's collection of sight glasses and see if one of these would work for you. Check out part number 1079K18. Be sure to verify that the plug is 1/4" NPT though.