Please take a minute to tell me if my plan is correct or not

Check everything... twice. Another member put quarters on his B'cuda and later found they were a tad too far spread. The rear bumper wouldn't reach/fit. Luckily that bumper has a slight bow so flattening to spread it a little made it fit. We can assume that's what the factory workers did too. Minimal adjusts are available everywhere but we gotta be within their range of adjustment. Aftermarket parts may or may not land within that range.
Take your time and screw it together before you weld it. Keep your eye on all the gaps.Use weld through primer when welding a panel over the top of another.