At the speeds that setup will be generating.. getting good flight w/ a fixed blade broadhead will be tough... esp a thunderhead or muzzy.

I shot my Patriot Dually at 70# w/ a 400gr (including 100gr point) Gold Tip 7595 at 308fps.... I got the best flight w/ 100gr Magnus Stinger 4 blade, Steel Force 100 gr, and Wacem 100 gr. Also might look at mechanicals.

A few that I'm going to try this year are Razorcaps and Grim Reapers.

Also you have speed to spare w/ that setup.. so I'l go w/ a little heavier arrow.. the heavier arrow will make the bow quieter and less critical to shoot.