NSArray and NSNumber-int conversions
You could consider creating a class that holds the indices for a larger amount of text (for example a paragraph or a section or a subsection in the text). That class would have the absolute index for where in the file it begins and relative indices within the paragraph/section each line begins. 

Whenever a new line is added or removed only the lines within that paragraph/section would have to be updated and the absolute start index of all the following sections as well. 

(I don't know what text you write or what you use the line starts for). 

You could still find a specific line very easily (and probably very fast) by enumerating the absolute indices of the paragraphs/sections and when you find the right section enumerate it's lines.