My '72 Duster is an original A/C car but I may consider an aftermarket kit to replace the stock system. The Classic Auto Air setups use a compact unit with the fan motor fully inside the car under the dash. The kit comes with block off plates but to me, they look bad. I want a smooth firewall or at least, something that looks cleaner.
My welding skills are limited but getting better. I am faced with maybe just making a sheet metal cover for the firewall and welding around the perimeter.
Any tips on what I should do? I'll have the seats and carpet out to avoid fires.

I converted a 67 Dart firewall from AC to non AC, got the non AC "plug" from the salvage yard and fitted it in as shown below...if I posted them correctly, first pic should be before, second pic during install and last pic when finished, hope this helps,

The stock under dash part is much better than the general one they make.

They also make "rebuild" supplies for the original a/c under dash parts.
The blower motor is better and the surface area is better.
You can just do the compressor portion and maybe new condenser and keep your under dash part if you can.

What do you plan on doing for heat and defrost?

I plan to use an aftermarket kit from Classic Auto Air.

if your going full aftermkt, I would just cut the old out and weld in a new flat plate to make a smooth firewall

Thank you, that makes the most sense.