Chris,
"They are having a hard time in the field believing me"............
Show them the Table 3.1, Group I, Steel Specification Requirement: ASTM A36 (for materials 3/4' thick or less)
Process: SMAW
AWS Electrode Spec. A5.1
Electrode Class.: E60XX, E70XX
Then show them the Table 3.1, Group II, Steel Specification Requirement: ASTM A572 Grade 50
Process: SMAW
AWS Electrode Spec. A5.1
Electrode Class.: E7015, E7016, E7018, E7028
Then show them Note 1 that states that in joints involving base metals of different groups, one of two filler metals may be used:
(1) That which matches the higher strength base metal
(2) That which matches the lower strength base metal and produces a low hydrogen deposit.
Since item (2) applies, that outta do it.