I used a slight bit of spray adhesive for the wheel well carpet pieces, nothing more than a tiny bit to hold in place. The side trim panels hold it mostly. You may need to trim a small amount near the rear seatbelt retractors if you have rear shoulder belts like in my 1993. I chose some spray adhesive because I didn't want it to move around while fitting everything.
As for the fold-over lip on the front edge of the rear floor carpet, yes it comes flat and not heat moulded on an angle. Just fold it over and it actually tucks in the corners near the lower front seat belt retractors and stays in place. after a few days, it kind of takes shape.
You mention that the rear floor carpet piece is too wide in the back? Be sure you remove your rear plastic trim plate and fit it under that, all the way backwards. The mounting screws will screw through that trim plate AND the carpet, ending in the body of the truck. For width, it is the proper size. You have to remove or move your rear side trim panels to get them out of the way. Then, fit the carpet into the back, all the way across and you'll see how it fits. Once your rear trim panels are back in place, they cover several inches of the rear carpet on the outer edges. Mine fit perfectly when installed in this manner.
FYI, the rear floor carpet goes in last, AFTER you get the wheel well carpeting on. The floor carpet is bound on the edges and lays over a certain portion of each wheel well carpet piece, which also helps hold those pieces down and in place.
Does this help?
Do you have any pointers for installing around the rear wheel wells? I seem to have a lot of extra carpet there.
Also did your rear piece have the 90ish degree mold to fold down over the step up in the floor? My rear piece is all flat and seems to be a bit too wide in the very back of the truck.