Thanks for the info Glamis and yes it's getting plated top and bottom to reinforce it all. It's exactly to a T a +4 and no way to screw or up because of how I had the blocks milled.

I would hollow out the machined insert to reduce the stress at  the welds.  The machined insert needs to have a similar amount of flex as the rest of the extruded aluminum side rails of the OEM swing arm to prevent stress risers at the edge of the welds.  Those solid machined inserts are not going to flex but the extruded side rails will flex causing it to break where the flexing stops.

I would make a heavy steel jig to hold the swing arm in alignment while it is welded.  I will distort while it is being welded causing the swingarm bolt alignment and axle to not be parallel.

I learned the above the hard way many years ago.