As long as you can ensure you are getting "P.U.I" branded Impala interior soft-bits, that is where most Impala retailers are offering. You can trust the P.U.I. brand.I've used C.A.R.S Inc as well, I like them. Can't recall if they were PUI brand or not.
I am currently having an issue with a set of front bucket seat covers that don’t fit in my original interior 67 SS. I ordered them through a very reputable vender (PUI brand) and have a very good auto interior guy installing them that I’ve worked with before. I’ll let you know how this works out, hopefully it’s just a one off bad set.
It will be interesting if you can pass along a nice description of what exactly the fitment issue is. Too narrow, too short, wrong pattern, etc.
I will say that I also had an uncommon experience on my first rear seat PUI covers I ordered, the pleats were a different width IIRC. They fit - they just looked wrong! Fortunately, the vendor looked at my pictures and was able to recognize that I had been shipped a very old set that was from a manufacturer's bad run of the product. They took them back and sent me correct ones. Granted, your situation sounds potentially more dire. Good Luck though!