I used mass backed ACC on my Challenger convertible a few years ago, and even though it was some extra work (and sore and blistered hands that took days to recover) it was worth it. I don't recall the sills being a huge problem to deal with. Cut a relief in the carpet piece where one half of the carpet overlapped the other, and had to do some trimming to be able to install the sills such that they didn't rub against the interior door panels when you closed the doors, but all in all it wasn't too difficult. Of course, with E bodies being popular they may have optimized the design of the carpets for them better than the carpets that they sell far less of.