the best way to display OS3.5 icons is to bought the OS3.5 or 3.9
In fact you can't convert most of 3.5 icons to NewIcon as NI size is limited to 93x93 and some GLOW are really bigger than this so i guess you've experienced some crash with convert35icon while trying to convert them. Perhaps you've been lucky and few of them are now in NI form.
You can still try on small GLOW icons... i wish you good luck !