A quick & effective way out of that one is by using 4 large steelwashers & Locknuts...i'm suprised the wood is as soft as you say!?.
Or you can reinforce the back of the bulkead/firewall with a 6mm ply piece & use longer bolts.
I would not trust to leave it like that..Saito 120's have bags of torque...it may work loose & pull out over time...a bad accident that would be!..