I used a stainless steel hose clamp to squeeze the two pipes together to support each other and minimize vibration modes. The clamp screw is safety wired. There is no brace to the firewall. No cracks after 2000 hrs.
My backup plan was to attach two ¼ inch tubular braces running up to bolts on the oil sump, providing a triangular brace that moves with the engine.
I was surprised to see how much a cross firing engine can move on its mount, rocker box covers can move well over an inch. I am not a fan of firewall mounts unless there is a flexible ball joint in the system.
My crankcase vent is on the opposite side from the exhaust pipes. The engine occasionally blows a slug of oil out the vent (mild acro screw up) and I don’t want it near a possible ignition source, the belly is easy to clean.