I can't remember where I got my f/g fenders but there was definitely some arm wrestling to get them in place. Ditto what britcon said, trust the holes in the bed as being accurate. You can either flex them in place or if that's too hard you can (a) make a series of stress relief cuts , shape it correctly then repair the fiberglass before painting, or (b) my choice, call it close enough after flexing it as much as possible and drilling new holes.

'69 1/2T stepside, Sea Foam Green, 250 L6 w/ 3 on the tree, not an option on it, in the family since new.