Here's my suggestion.

Square up the firewall so it's exactly where it started. Make a new firewall that matches the original exactly, you can fit it in back to get the right size.

Epoxy 2 1/4" square strips of some hardwood like bass in the corners behind the original firewall. Then epoxy the new one behind the strips, using the strips to maintain the original angle so now you have two identical firewalls 1/4" apart.

Cut off the front one.

Voila. A brand new firewall spaced 1/2" back.

Mount the engine with your origional standoffs.

Now cut off the back edge of the cowl to shorten it and redrill the mounting holes so it's about 1/4" behind the prop.

This shortens the fuse by 1/2" which eases the node heavy condition and give you a shiny new firewall.

TF