Engine firewal needs to go

I need to pull the nasty engine firewall off the sides of my 68? But don't want to replace them until I finish wiring it up. How necessary are they? Ive had numerous bugs without them and incurred no problems are they for sound, heat, airflow, all of these?.

Pretty much all they're for is sound deadening and heat. That flat 4 whine can get to you after a while as it echos and reverberates off the bare metal of the firewall.

It'll also prevent engine heat from radiating through the cabin.

If you can live with the whine and the extra heat, there's no real reason to replace it. Most show-bugs I've seen just have the firewall painted to match the car with no insulation.