Currently converting a F-1 with a IO550 to a dual SDS system making this the 4th EM-5 install for me. I will be mounting the regulator mid height on the firewall, dictated by the limited space and existing plumbing.
Your regulator can be mounted anywhere it makes sense to you, I have installed them high & low dependent on space/heat/plumbing restrictions. The return line eventually routes back to the duplex fuel valve, so having both firewall bulkhead fittings close together makes sense (to me). The lower firewall is usually less cluttered than high, so it makes sense (to me) to mount the regulator down low, away from heat sources, providing there is a good path for the flex fuel line to run up to the back baffle area. Also I like to mount a large capacity inline fuel filter on the firewall on the feed side, this insures no debris from a failing fuel pump would contaminate the injectors. So the regulator & filter could be close together to allow parallel fuel lines running up to the injector manifold mounted on the top of the engine.
Time to slow down the Builder Assist operation & concentrate on finishing my own stuff & fly the 6A.
My 'retirement' activities list;
Mon - 6 build project to finish
Tues - 6 upgrade project, canopy, panel, interior & fairings
Wed - 9A salvage rebuild, wing, firewall, gear, engine
Thurs - 7 salvage rebuild (was second plane I built, the Turbo Subie) for a buddy
Rest of week - belongs to the wife!