I am trying to determine the factory length of the (r) and (l) lift arms, sometimes called a leveling box. Once on the right is adjustable and the other is not. Specifically I need the overall length of the part number T6146-39100. My local tractor parts store (Tractor Supply) list two different ones but no model of tractor that they fit nor which factory part they replace. so which one would I need to replace the left (non adjustable) arm?