More trailer work prior to welding

The suspension is a small leaf-spring that attaches to a couple pieces of painted steel angle brackets.  It is this bracket that I will be bolting to the aluminum frame.  To avoid aluminum / steel contact, and to provide some spacers, I added UMHW plastic shims (white) that are rated for outdoor UV exposure.  

The axle needs to be positioned to give 10-15% tongue weight based on my extensive education watching YouTube (LOL), so the holes will be drilled after the whole trailer is assembled.  

There will be two holes in the shear web (less critical) and one hole drilled in the bottom structural cap.  Drilling in the caps is more detrimental to the structure, so I want to minimize those and put any in places that are less stressed.  For the trailer's loading, I expect the upper caps in tension to be more critical than the lower caps in compression, so I'll only accept holes in the lower caps.  Further, the overhang aft of the wheels seems more stressed than between the axle and tongue, so I'll avoid drilling into the lower caps aft of the axle.

I made some wedge shims to make any bolts going through the caps have a flat surface to tighten against.  These are cutoffs from the end mitering with a hole step-drilled up to the right M10 diameter.  Six were needed for the hitch bracket.  Thinking ahead, several more of these can be welded into the cross-members in places where I'll bolt down the decking (I'm still thinking marine / outdoor plywood to get over a couple seasons).

Thinking ahead for the decking, I bought some angle 6061 aluminum to weld onto specific cross-members so the plywood decking has somewhere to rest its edge (4x8ft, 4x4ft, 4x8ft).  These should be easy to weld in place.

I also bought new 316 stainless metric hardware to replace whatever was rusting on the old trailer.  The aluminum / stainless is unavoidable, but I am minimizing contact where possible to try avoiding galvanic corrosion.  Anyone have a suggestion for a grease or coating that I can spooge / spray over the area?

I did talk with a work colleague who is a professional welder, so one path toward completion is opening up.  There is some work on my end remaining prior to welding still, so no action other than making the contact at this point.

The tail light brackets were quick.  They do still need UHMW spacers between the pot-metal steel brackets and the nice aluminum frame.

Work prior to welding:

• Drill holes for wiring to pass through the ladder rungs' webs.  It's much easier now than later.
• Mount the trailer tail-light brackets with a UMHW spacer for dissimilar metals.
• Buy & mount the amber side-lights.  This is probably fine after welding though.
• Make additional wedge shims and position those for welding