You will probably get a lot of gaps and thus air infiltration if you try to fit rigid insulation between the trusses. Why not try using blown cellulose rather than the fg? Best to have this professionally done. Nothing good is cheap.
I agree with the likelihood of gaps in the foam board but you can use an expanding foam to fill the gaps.
Breathing of Fiberglas is a temporary condition during application and can be affectively avoided with proper apparatus.
3M makes several good Respirators that are rated for insulation. http://products3.3m.com/catalog/us/en001/healthcare/animal_care/node_12MQL10JDBbe/root_GST1T4S9TCgv/vroot_PZ8MXGQSQDge/gvel_L0RQLML15Ngl/theme_us_animalcare_3_0/command_AbcPageHandler/output_html is the EARL (URL) for more info.
As is pointed out about once a week in this forum, seal all penetrations thru the attic (electrical, etc) - or you'll be breathing far more insulation in the house than you ever would installing it with a mask - AND loosing more energy.
I don't insult myself, but I've been itching to try (pun intended) MIRAFLEX - it doesn't itch. the Owens-corning site doesn't mention the danger to breathing it. The site also has alot of good info on insulation.