I have 1450 w/44A deck.  Pretty sure the configuration is the same for the 1250, 1450 and 1650.  I use a 1/2" x 79" Kevlar belt form TSC.  I buy 2 of them at end of the season (when they are discounting belts).  2 last me all season.  I use my mower for cutting horse pastures.  Sure that is the only reason I go through 2 a season.  Pretty hard duty on deck and machine.  But it's been doing this level of service for 12 years and still going strong.  Love this machine....