17 with 1974 duster 225
Nice looking car!
Well, I'm not a big slant 6 guy, but right off the bat I'd say that's not an AC car originally. Not unless someone filled the holes for the condenser. Here's a stock AC firewall for a '74
Here's a '70, it's a little different for condenser and heater core locations but you get the idea, you're missing the condenser holes either way...