Hello scott.va, according to information that I found in one of the Alcoa books. You can use 5356 filler to do this with good success according to their rating chart for welding these two alloys to one another. You can verify this by obtaining a copy of "Welding Alcoa Aluminum", Copyright, 1954, 1955, 1958, 1967, 1972 Aluminum Company of America. There may also be newer versions of this information or others, this just happened to be the one that I found this information in. Best regards, aevald