Justification statement for exceeding the maximum length of manuscript

Assuming it is true, I'd say that I'd made a good faith effort to keep it to the limits, considering whether any of the material was extraneous, but it is still at 37 pages.

But also add that you would welcome advice from reviewers including advice on reducing the length.

I doubt that a detailed technical statement will get the result you want, since the editor may not be able to evaluate what you say since that is more the job of reviewers.

Some papers can also be split into two, of course. Perhaps you have considered that already. Not all, of course, but it is worth a look.

Perhaps most important is to explain why this paper belongs in this journal, and not in another journal that allows longer papers. I am not sure of the research area, but in mathematics there are journals that allow much longer papers than 30 pages. What is so special about this journal?

If there is somehow not a more appropriate journal, have a look at other papers in the journal that are over in length. Perhaps none are longer than 32 pages, and yours has no hope unless you trim further. Perhaps there papers as long as 37 and you can deduce the reasons. If there are lots of papers well above 30 pages, then your colleague is correct that this is a minor issue. In that case, do as Buffy says, and explain why this cannot be sliced into two 25 page papers, all the effort you put into making as short as possible.