Need the correct belt

I am working on a 1973-1975 John Deere tractor and cannot locate the correct deck drive belt. John Deere tractor numbers (C300D---030562M), and deck numbers (HO48L---470385M). I have two belts in hand, one is too short and the other too long. M45254 is 86.3" and M82718 is 90.3". I am looking for the belt which runs from the PTO to the deck. This should be simple, but I am missing something. Owner is refurbishing unit and wants as many OEM parts as possible. He has had the unit since new and the deck came with the unit. Yes, I have tried adjusting both the deck and the mule. Thanks in advance, Rivets.