Here's a firewall with the main part cut our and a replacement fab'ed using a bead roller to put 'factory like' features back in it and to better clearance the engine.
The time to modify the firewall with a new panel would have been about the same as filling all the holes of the factory firewall and modifying it to clear the V8. Here it is at the fabrication stage before it was welded in-
The remaining holes in the firewall were filled using gas and bailing wire. Bailing wire is very low carbon and is still very ductile after welding so any shrinkage from the weld is easily stretched back out with some hammer-on dolly work. It also grinds very easily. These filler plugs are non-structural so I don't have any problems using a non-standard filler rod.