Wyo, my zenoah mounts in a similar fashion and it was a pain getting the firewall drilled on the H9 for the top bolts. If you can still do so, pull the cowl, drill, then mount..my top bolts, as a result of being an ARF have an angle to them. I don't like it, at all, but short of cutting the firewall out completely, no way around it.
I dunno if you can see it in this picture, but the plate is nearly as big as the firewall. Upside, its strong as #$%@, downside, I wish I could have drilled the firewall out of the fuselage. Something I will do before gluing the firewalls in the
Lightning. So if your upper cowl isn't glued yet, tape it in position, mark holes, pull the upper cowl then drill and mount the plates. It will be much much easier.
I could have went straight through the firewall to the engine, but its actually a ptetty narrow pattern, three of the bolts would have went through the ply filler plug..was my origional plan till I got a good look.