If it were me I'd go for option B. Trim it up nicely and weld maybe a piece of 1/8" flat bar. Have it wide enough so it spreads the load out more. The worst part is welding from the bottom.
That crossmember supports the floor, too, so I wouldn't replace it with something which doesn't also provide floor support. It's a bit difficult to make out the shapes and contour of the crossmember, but it looks a bit complex, with a convex shape to the underside where it transitions into the shock mount area. I would replace it with either good used or reproduction. It appear the LH mount hole is patched over, and someone didn't have the proper shock tool on hand, so they took a cleaver to the topside access hole, too? Yikes. Are their drain holes at the forward end of the bed's, well, underfloor?
We had a ford do that. Dad but a piece of plate on top of the crossmember. Put a shock mount in the plate.
Suggest "B", only put a plate on the bottom, but instead of welding from the bottom, first drill 3/8" holes in the existing sheet metal & plug weld to the new plate from the top. Would be a LOT easier...
Tri five Chevys have similar issues with their rear shocks, especially when someone used air shocks to clear their N50-15s.... Someone make a bolt in shock cross member kit for them but those are full framed cars. If it were my car I'd cut out as much of the damage so I could to make a square, circular or rectangular patch to drop into place and weld it in and any of the traveling cracks. Luckily it's not in a highly visable area. If you really wanted to you could open up the top hole so you can get at it since the bed floor plate covers it.
I just checked for*****s and giggles to see if they make this panel as a replacement for Mustangs... and sure enough they do: https://www.cjponyparts.com/rear-shock-mounting-panel-1965-1970/p/RSMP/ Looks a bit different at the floor mounting but you might check it out...
The Jalopy Journal
