the reason 4043 was used on the part in the first place is because 6061 T6 aluminum is strong, but brittle. it will not bend or flex very much without stress cracks forming. the 4043 makes a soft and flexible joint. if a rod w/ a higher tensile strength than 4043 is used, you risk the chance of having a joint that does not flex at all, and cracks may form at the toes of the welds.
5183 (Al-Mg) filler metal is generally acceptable for welding 6061 (aluminum, magnesium, silicon base alloy). This is just a general application, I do not propose that this is specific to your exact application. Hope this helps. (Sorry for the disclaimer)
Respectfully,
Mike Sherman
Shermans Welding