Make sure the proposal is constructible, if you are being asked to design it.
If wall is cantilevered, conservatively design it for active pressure plus pressure due to possible surcharge (which if this is temporary, should be the normal 2'-0" surcharge), whatever the condition of the fill will...
The key to your problem is properly modeling it.
The field welds are indeed 3/8" per your sketch, not 3/16". And since this is existing, check the column for an applied (limiting) moment, from all causes (dead plus lateral, applying proper factors depending on how you are checking it - ASD or...
If there is no problem extending the bottom plate to the right of your sketch, extend the column up; provide a column cap plate doubling as the top moment connection plate for the 3 beams; provide a bottom moment plate extended on 4 sides. Design the width of the plate extending to the right of...