I think you may be pushing your luck with straight bolts at 10cms long? Have you considered maybe 6mm dia bolts set inside aluminium tube spacers - say 10mm - 12mm OD?

How about a transverse length of aluminium box extrusion - one side bolted to the firewall and the engine bolted to the other side? Something like 100 x 100 box section with 3mm wall thickness. You could carve great lightening holes out of all sides to keep the weight down?

Good luck,

Alan W