Firewall/Heater Question

[Old Chevy Lover]

I have heard some people talking about filling in/smoothing their firewalls. I was wondering if they just do away with the fan and heater core, or if they move them to the inside of the truck up under the dash somewhere... It looks like I could fab up a place for the heater core in the ducting, but still would have to have the fan on the firewall. Just looking for some ideas.. My firewall needs a fair amount of work where it has rusted up at the top lip. Figured while I got it all apart, might as well make some changes