You should be able to reach something like ~345 with the fh21' at Monza with kers deployment + drs on main straight (gear ratios are, of course, fixed as per season rules). Unless you're forgetting to reduce aero from the default setup...
I don't know if you're new to AC, but unlike something like CM F1 games, by default you only have a generic/safe'ish setup for your car. I.e., a session doesn't automatically start with an adequate low aero setup on the fastest tracks, that's something you should work on (then save, so you can load it later).
PS: Btw, current fh21 is v3. Download it again from Sellfy.