Ultimate steel wouldn't even be a consideration. Blade retention sucks.

Two fixed blade headsI would strongly consider on that set up. Wasp BOSS Bullet 75 grain and Slick Trick 85 grain. The ST being my first choice.

Both of these two heads SHOULD provide you with excelent flight at those speeds.

I think the 75 grain Muzzy would be really iffy at 350 fps or faster.

If I were going to use a mechanical it would be the 75 grian WASP.