26AC/HK are not released for online check-in. The best you could get would be 26D/G (they do have a bit of a legroom for your left/right foot). Also, if there is not much movement between C and M cabin, you could try stretching your legs into the C cabin if you sit in 26D/G (but then be also prepared for occasional cart bumping).
Alternatively, if you live close to the airport, you could show up at check-in counter 24 hrs before the flight and get emergency seats there. This will only be possible if your A346 flight either departs from the airport where you are located or, if your traveling involves a connecting, if all segments are on one ticket.
(I once flew LWO-MUC-FRA//separate ticket//FRA-KIX). The agent in MUC could see the seat map for FRA-KIX but because FRA-KIX was on a separate ticket, she could not change my seat assignment even after I have done online check-in).