I flew on a DL 753 last year with a Graco Nautilus, which is 18.9 " wide. The published seat width on the 753 is the same 16.3-17.3" you listed for the 738. I do recall my wife had to keep the armrest up.
I also had a Chicco Keyfit 30 that we bought specifically because it fit in my wife's Kia Spectra, I highly doubt you will have to worry about it being longer than the seat pitch.