The only thing I can think of, is typically, for undermatching weld metal, the weld metal controls the strength of a joint. For matching weld metal, such as those in Table 3.1, the allowable load on both the weld and the base metal are the same. For overmatching weld metal, the base metal is the controlling variable.
Your situation of 80xx to A572 Grade 50 is not in Table 3.1 for filler metal matching strength, so to me, it would fall into the overmatching category, in which the base metal is the controlling variable. That being said, I would think that you'd have to use the 1 1/2" for the "specified" 50 ksi and under, since the criteria for a bend test is based on either the specified base metal strength OR the actual base metal strength, which can be taken from the MTR. However, the actual MTR yield value will most likely put the base metal into the 'over 50 ksi" criteria, which would then require the 2". As far as I know, the bend test criteria is without regard for the weld metal strength.