Hello there, jwooldridge and welcome to the forums.
Can I ask, before we go on, what code are you welding to, and are you using a properly qualified WPS?
Aluminum is a tricky beast on the best of days, and if you're not prepared for it, you'll find yourself doing a lot of rework. I wonder what the intent is in using the 5356 filler...could it be that these parts are going to be clear anodized, and the customer doesn't want discoloration? In this instance there is little that can be done, as the anodize process will cause your heat-treatable fillers (4043, 4643, etc.) to turn somewhat black. In any case, good luck!