Suspension and frame work, cracked LCA, advice welcome

It is unusual....THe sway bar tabs are out to the edges of those LCAs like the early A body UCAs of American heritage.
I'd fix what you have. Welded metal can be even stronger than the original steel if the weld is done properly over clean steel.
Yes that is the only LCA that was used in the Brazilian Darts from 1969 till 1981.