Dave,

 

Here's another way.

 

Cut a piece of 1/8 inch plastic sheet (window replacement material from hardware store) to the shape of the firewall and drill the engine mount bolt holes thru it.  With the engine suspended by its lift point and the engine mount attached to the engine, the plastic can be moved between the firewall and the engine mount as a see-thru template marked up with a permanent marker.  Firewall located equipment can be test mounted or located on the plastic (mounting holes drilled) and, when you are happy with all the component placements, the plastic can be used as a drilling template for the firewall.