Here's some more detail. If your concerned about the Glue setting up on you,just get slower drying Glue. 90 minutes is plenty of time. I don't like to weld skin's because of the warping and I think it takes way to much time.
Golden Legion sheet metal is what I prefer. It seem's to fit the best.
I use 3/4" Tape for my cutting line. I use a standard cutting wheel. The thinner the wheel the better. I cut the top, front and rear of the quarter panel. I loosely cut around the lower rocker,wheel well and drop off.
When the panel is off, I remove the old spot welded metal from around the rocker,wheel well and drop off area. What you have now is a bonding strip.
Take the new skin and remove the flange from the front, top and rear. Take the skin and trial fit it to the the quarter area. You may have to modify the flange that sit's into the rocker area. I've had to re-bend that flange often. It may throw your skin alignment off. Make sure the wheel well area's fit. Use a Vise grip at the center of the wheel well area to help you steady the panel. Wear glove's, the metal will bite. I use sheet metal screw's at the front of the panel. Tape at the top and clamp's at the bottom. Practice until you get a routine going on installing the Skin. It'll get you familar when you do the actual install.
Once everything is fitted,remove the panel and prep the areas to be bonded. Keep the prep area on the skin about 3/4" wide at the top and side's. The same width at the bonding strip you made on the Mustang.Prep the area quarter area on the Mustang. The area must be clean.
The Glue will flow better if the tube is warmed up. If not, you will probably bend the Gun and your hand's will get tired real quick. You don't need that at this point. Use a slow drying adhesive.
!Read the instructions
Have all your clamps, vice grips,sheet metals screw's and tape ready at the Mustang.
When your ready to go, squeeze the first couple of inches of Glue on the Wheel well flange. This will purge the mixing tip of poorly mixed epoxy. With that done, just apply the glue on all the bonding area's as required.
Once you have applied a bead of glue to the bonding area's, take your panel and PLACE it onto the bonding area. Do not pull the panel off, but you can slide it. Use a vice grip to the center of the wheel well flange to help steady the panel. While keeping the panel flat with your hand, install your other clamping device's till the panel is in place, then finish installing the rest of the clamping device's onto the panel. If you find small air pocket's,fill them with the excess glue. Do not clamp down to hard. You don't want to squeeze out all the Glue. Use your finger to smooth out the glue at the rocker seam. If needed, add glue to fill gaps. This will give you a factory looking seam.
If you need to install more sheet metal screw's, go ahead and do it. Getting the panel flat is the goal here. Be flexable and do what's needed.
Give the adhesive a couple of day's to cure, then remove the clamping device's. Take a grinder and clean up all the edge's. Glue, overhanging metal,ect.
Counter sink the hole's you drilled for the sheet metal screw's. Then apply a skim coat of filler to square off the edge's and generally smooth out everything. Most new sheetmetal need's some body work to get them straight. Go into the trunk area to remove the "Bondo Worm's" that squeeze through the sheet metal hole's.
Apply primer and block out the panel. I use a high fill Epoxy primer.
Then of course prep for paint.
I did this on my own Mustang years ago. The repair has never failed or has given any indication of failing. No complaint's from the owner's who own Mustang's that I did this repair on..... and I've done plenty. No one can tell the panel was replaced until I tell them and they just about break their neck looking up in the Trunk to look for the bonding strip. I've also done this repair on floor board's. Do not used bonding adhesive on Structural Area's.