Chas:
I’m with Dipper_Dave on this.I’ve got a Phd in Physics, which involves a lot of math (cue switchlogic).
Your 3000kg load could easily become as much as a 30 000kg+ of FORCE (note . . force!) during heavy braking, an emergency stop perhaps, or (shudders) a head on collision.
Your puny straps will not stop it moving & it will laugh at your pathetic headboard as it smashes straight through your cab.
Tell the boss it’s alright his way, provided his missus & kids are in the cab with you.
Failing that, do as Dipper_Dave suggests. Chock those wheels & use chains to tie it down.
would that not be slightly over 10g? an f1 car can generate 5.5g under braking and a veyron 1.3g apparently