You can keep everything you’ve got except for the calipers and hoses. Replace the calipers and hoses with a set intended for a ‘70-‘74 Challenger or a '71/'72 Plymouth Satellite and you should be fine. Those calipers have a larger diameter piston than the 73-75 A-body calipers did, and those brakes in good working order should stop your car quite well. There are some folks that like the pin style better, the sliding surface on the caliper brackets for the slider style calipers can get pitted and damaged. If that happens the calipers can hang up, and you either need new brackets or you repair the surfaces. With the pin style you just buy a new set of pins.
The caliper you need to work with your current pin style caliper brackets looks like this-
View attachment 1715388161
View attachment 1715388162
View attachment 1715388160
You can see the line is offset to the side instead of being right in the middle like the 73/74 B-body caliper. Hopefully these pictures can help you ID the calipers you need, we all know that just because you specified a certain part doesn't necessarily mean that's what'll be in the box.
As for the lower control arm, while I'm not a fan of that notch they cut the placement of the notch is in one of the least critical areas for load bearing on the LCA. It's ugly, but it shouldn't significantly weaken the arm. If you can weld, it would be a pretty simple repair to weld a patch back into that flange. As long as the rest of the control arm is in good shape that's what I'd do.
