I rebuild for best results -and try to ignore the easy setup.
It is like buying a cheaper suit - --You may have
saved a buck but you will ALWAYS wish you had done the job right the first time.
The engine must get a full ,straight flow of air to the carb - so make sure there is a hole bigger than the carb inlet - behind it - OR keep the firewall at least a full carb hole dia away.

My favorite setup -for all my rear intake engines (and who would want any other setup?)
is a 1/4" plate mounted to the engine - then a standoff (boxing) back to the best place to attach.
So - the standoffs are all outside the engine bolt pattern.
In this case - cut a huge hole in the H9 "firewall" because the standoffs only need to attach to the edges.
What possible good is a big slab of plywood between the mounting points
Ballast?