What is outside circumference of the old belt? Someone may changed the flat idler to a none spec one. But with the old belt OD should be able to convert back to the B (5/8) belt. But according Encore the B78 belt is the correct belt.
Also measure the new belt you got as I have belt spec'd is not the belt you get. I had lots problem with belt being shorter than spec'd. I just had that problem on a JD last week where the JD was to 141.7" but the new belt from JD was only 139"