On this thread:
(Safety Chains versus Cables)
on/about post 409 or 50 are links to a couple of types of safety chain attachments.
The thread begins with a dicussion of cable versus chain, and I have added several quotes on legal requirements. Prominent is FMCSA 393.70 which states, essentially, that
Of a pair of chains (or cables) EACH must be of a strength sufficient to handle the GVWR of the trailer. Same for each attaching end on the truck and trailer.
So, for my trailer that would be  each 8,000# points, and  Grade IV chains with hooks, etc.
The question here, specifically, is about how to specify the best possible welding on the trailer tongue and to the truck.
On other forums I have read things such as: "Ah, I think I'd use maybe stainless 310 rod. Or 312".
I have no experience to judge this with. Any good advice, or links to welding practices which educate would be gratefully received.