For what it's worth:
length of the outboard edge of the driver's side head on my 1:1 '91 Mustang's 302: ~19.5"
length of the outboard edge of the head on Revell '32 Ford: 19.54mm
length of the outboard edge of the head on AMT '67 Mustang: 18.65mm
BUT, if you compare the two heads, the "lump" on the ends of the '67 head don't extend to the edge where I was measuring like they do on the '32 head. I couldn't even really see the ends of the head I was measuring on my 1:1 for all the accessories, wires, lines, and poor light, so I don't know if my 1:1s have the same "lumps", and if I was measuring the lumps or not since I was measuring by feel just above the header tubes and below the valve cover. With the engine in the car, I can't measure much else on it...