To gitto and uncryptic, and anyone else struggling with Grid B.
Following the preamble, you want a systematic way of associating each digit from 0 to 9 with either two or three letters of the alphabet, so that each of the 26 letters is associated with one and only one digit. By "systematic", I understand "capable of being expressed as a simple rule that applies to all 26 letters".
There is an obvious (at least when the penny has dropped) way of doing this that allows Grid B to be completed according to the instructions. Grid B as a whole then represents a thematic word that would be encoded to the same sequence of five digits as the encoded version of the puzzle's title.
I'm sure that you will get the systematic cipher in the end!