On my personal; experience i have installed 2 diferent ones and looked at items from 4 places. The fiberglass deck lids and end caps are about the same and all required fitting and finishing to achieve a presentable image. The most work it's at the end caps. They don't have a perfect gap at the lid and the glued studs normally come off. The lid has some warping at the rear near the center and must be filled evenly with some Mud. The lock mounting also has to be worked out a bit and the rod has to be changed or modded.
We used Mier's racing spoiler/endcaps on our 65. If you do a search on my user name you will find a write up i did on fixing one problem we had. As ARPM said a lot of work will go into the end caps. the problem I wrote about dealt with the deck lid. We did not have any warping, (other than the back of the spoiler was not straight) but when mounted, we had a nice gap on the passenger side, but large, .5+", gap on the driver side. Additionally, there were a significant number of large voids under the gel coat, some as long as 12". I added ~.25" to the driver side. Their isn't any FG that works well. On our FG Shelby hood, had a reasonable fit to begin with. After painting, the hood was off the car ~ 1 year. When we remounted it, it had bowed on the sides ~1" and across the front had warped about the same. We ended up making an all steel hood with a steel shelby scoop (also on another post with pics). In the end we spent just a little more $ on the steel hood than the FG version when shipping is considered, much less the anoyance of having to redo and delay.
I molded my end caps one for a clean look. I got lucky with the fit, it was no that bad. Got mine off ebay and the guy said it was a maiers he thinks. I also smoothed the lock hole and put an original style remote trunk latch