Building new Masthead sheave box.

I would like to build a new masthead for my 40' sloop. The existing masthead is built in welded stainless and has two small wire sheaves. The mast is tapered to a roughly 5" x 6" OD end with 3/16" wall thickness. There is a notch cut fore and aft for the two existing sheaves.

I would like the new masthead to have four internal sheaves as well as spinnaker crane. The new sheaves (Harken 712's, I already have them) are much larger (4") than the old ones.

Designs seem to be pretty standard with some mastheads sitting above the extrusion, and most sitting within it.
I'm debating the choice of materials. Stainless would be strong but heavy. Alloy seems to be much more common these days. A casting is out of the question. I can design a masthead without any welds if I can find a good source for Alloy extrusions.
There are plenty of T-top welders around here, but I'm wary of weld strength on such a critical item.

Here are a few pictures I found for ideas.
Here's my design so far
Sparcraft 1
Sparcraft 2
Z Spar
These seem to be based on custom extrusions, but I could see copying them with angle extrusions and some non-structural welds.

I know exits must have fair leads to avoid chafe.
Any thoughts?