Putting a 70 Dart together

Question for all of you gurus. I'm assembling my 70 Dart and am now focusing on the rear of the car. I have my bumper back from the chrome shop, valence panel painted, and rear bumper brackets painted, even cushioned with the proper round white gaskets between bracket and car.

Should I attach the brackets to bumper first and then attach it all as one unit? Or should I tighten down the brackets first on to the car, leave the valence off, and attach the bumper to the brackets afterward? Will I have room to get in and under it to tighten everything? I could always put the valence on last.

Just curious as I've never built one of these and I want to do it right.

Thanks.