ClassicCar

I'm pretty sure the fenders are supposed to go on first. It's common to remove the grille for service (to change a timing gear, remove the radiator, and so forth) and you certainly should not need to remove the fenders in order to do that.

It is indeed tricky to get the sharp bottom edges of the grille seated properly in the space between fenders, without scratching the paint. I seem to recall that one can loosen the bolts on the horizontal rod that ties the fenders together, to give a bit more space in there. But if not, it's just a matter of taping some towels, blankets or thin pieces of cloth in there to protect the paint on the fenders, before you even lift the grille in place. Or even forming some thin cardboard in place so it acts like a shoehorn on each side of the opening. (This would be good if you have that heavy rubber welting pre-attached to the fenders. It will allow the lower edge of the grille to slide past the welting into place, rather than binding on the welting.) Once the leading edge of the grille starts to seat, you can yank the cardboard or cloth, out.

The radiator should probably be loose (don't install the braces to the firewall, nor tighten the radiator to frame bolts, quite yet, so that you can tilt the radiator to properly seat the grille). You need all the flexibility you can get, so you don't want a fixed radiator.

As you lower the "nose" over the radiator, a second person should be pushing inward on both sides of the grille and guiding it over the edge of the fender openings. Looking down, it's a triangular shape, so you almost have to seat it as far back as you can, then slide it forward toward the front fenders.