You can see the string in this picture, I used it to pull the Brim down while the resin setup on the outside.
I will be finished fiberglassing my helm today and moving on to final detail before paint. Should be done by Sunday. All said, the Helm, arguably the most recognizable part of the cheif, should be detailed and as accurate as you are able to make. Your armor also should, imo, reflect the quality of the rest of your work.
Do what is most comfortable to you. Correct me if I am wrong but The H2 Helm and H3 Helm are the same helm, both MK VI, just the game quality detail used in the H2 and H3 pep is slightly different. The Helm is roughly the same outer shape as far as I can tell also. So if you allready have 2 helms, start with what you've got, move forward. Add the details to the outer shell once glassed. If it's not what you expected, then you will have gained build experience along the way, and your next version will be that much better.
EDIT: I look like a nerdy advertisement for Sprint and Treo.... what a goon....