I'm not rigger, but where possible I'd go for aluminium spreaders and aluminium spreader mounts on an aluminium mast
. Dissimilar metals such as aluminium and stainless do not play well together, so keeping them all the same is good. Make sure you use a jointing compound such as Duralac or Tefgel wherever you use non-aluminium fastenings in your mast - stainless bolts & screws, Monel rivets etc.
As to what grade aluminium to use... probably best ask your aluminium supplier. 6061 is a probably reasonable, if you can get it.