I'd heard a few bad install stories from posters on tru-flex covers, but those were mostly from first-timers.
I think both fiberglass (buy from a reputable manufacturer) and tru-flex units are OK as are the GM urethane covers they are making now. The old urethane was subject to waviness when exposed to temperature changes.
My fiberglass covers went on after a lot of trial and error and by using the new style retainers. I went with the glass covers because I wanted to mold them into the quarter panels and fenders. (Filling in the seam between them.)
I found the easiest way to do the install was working from the center line out. I lined up the center line of the cover with the centerline of the hood, marked, drilled and worked to the outside from there. You can use a similar plan in the back by extending a (imaginary) line from the center of the fuel fill opening.
I used the tru-flex on my vette when the body was redone several years ago...just a matter of trial and error to get them to fit.Just bear in mind,you need to be much more careful as they do not take hits all that well!!!!!!