Looks like 3 1/4" standoffs could flex somewhat, adding to the vibration..Best thing would be a 1/4" aluminum plate between the 3" standoffs and the firewall, adds stiffeness......Cut the center out of the plate to make it lighter...]
Haven't heard of DAs breaking the legs off, but the Chinese engines do....And the very early 3W 60s and some twins, before they changed the design....Fixed a few....