Placement isn't critical. As long as there's enough meat left to keep it from pulling through the hole you should be fine. Hopefully you don't have any problems with rusted bolts that strip out or "spin" the cage nuts --- I'd recommend soaking all the bolts in some sort of penetrating lubricant as best you can get it on them and let them stew for a couple days or so before you start. I'd recommend you do one side of the car at a time. Take the bolts loose on one side, use a couple of jacks to tilt that side of the body up just enough to take the pressure of the biscuits so you can slide them out and replace with the new pieces. I mean just barely enough -- the fronts of the fenders connect to the radiator core support so if you go crazy lifting one side of the body you're going to be putting all that into a bind and will likely twist/distort something up there. Let the jacks down and tighten everything up. Lather, rinse, repeat on the other side. Keeping one side of the body "tight" at all times will preserve it's alignment to the frame while you're working on the "other" side.