Agree that MTD products needs only MTD belts installed because the unique belt design the MTD uses. But in this case I have used B38K belts in place those B x 41.25" belts. I even use up to 5/8 x 42 without problems but when it comes to the engine to variable it is only the MTD 9540467A. Usually although the belt looks good to the untrained eye there is a lot of wear. Auto Drive versions of the CV setup are more sensitive to belt length and slippage issues.
Also the idlers pulleys needs checking. I even replaced a few engine pulleys because of wear of the v groove.