Hi bud, I have done a few variations of kits and have gotten most of them from Facebook marketplace and repaired them if needed then fitted myself, however in terms of roof spoilers I have had both the HIC and non HIC ones similar to those you can see on GMC Autostylings website, I currently have on the non-HIC one with the vortex roof spoiler and they fit well, however there is a long wait time from that specific seller.
For fitting the Spoiler itself I de-greased the glass, applied silicone sealant to the flat spots lay it on and taped it down to allow it to cure, if you dont plan on changing it then I would suggest a generous amount of sealant, if you get one that lays perfectly to the rear window you could also use 3m tape. Remember to lay it to the window first and if you want a perfect fit, sand down any high spots and filler up any low spots, all simple stuff to do just time consuming.