Even 5058 will work. Its still a marine grade . Just soft and not as corosion resistant as 6061. Oh yea it polishes sweet but dont take to anodize for crap. And from my experience it will all weld, but very in strength. It should be easy to find 6061, you can still order plates from Bergeron and I bet Rex could hook you up.I just looked them up from Bergeron,3/16" go for $120 a pair and 1/4" are $140. Call them and they will make what you need.480-834-1531 hope this helps.
Mike