I've just found your query: we don't think it is tricky: you just have to use MSMS!
We wrote a software tool specifically for this task and to automate it: "Supernovo". IgGs in particular can be sequenced using a pseudo-scaffold, so we've piled in a whole lot of knowledge, and we re-run and re-run until we get good agreement. If you have the data, you can try a free demo for 30 days.