Do or DONT fiberglass body parts

My personal opinion only......
If it's a street car, and standard weight (not seriously lightened in other areas) I would stick with steel.
If it's a serious street strip car, where a couple tenths of a second mean something, and it's been lightened in other ways.... sure, go for the fiberglass.
From a resale viewpoint, the above still applies. If I'm buying a street car from you, I'd want quality steel (but not necessarily perfect chrome bumpers).
If I'm buying a car to race, light is right, and glass is good.
As for the glass you are looking at, I would start with the hood, most weight reduction for one piece, then bumpers (lots cheaper than new chrome here. LOTS!). If you have a Valiant, then yes on the deck lid, barracuda maybe not.
Fenders, to me, are a no. The weight savings is small, the grief is large, and so is the expense.
To me, fiberglass makes sense on a light car only. Save two hundred pounds on a 4400lb hellcat? So what. Save two hundred pounds on a 2700 lb fox mustang? Now that makes sense.
Again, my opinion only.