I have completed the grid but can see only 49 - and not 51 - cells that need omitting from it !
My set of six thematic entries (the initials of which are C x 3, I, N & S) total 29 letters, whilst my three single sets (H x 2, P & W) total 24 letters. I appreciate that makes 53 but 4 of the squares those entries occupy are shared !!