What 440 is saying sounds correct, although there were some aftermarket aluminum 318 4 bbl intake manifolds. I should know this, but I don't.... what size are the intake ports on a 273 engine? Weren't some of those 4 bbl motors?
As I recall, the 340/360 engines have larger intake runners/ports on both the intakes & the heads than the 318 engines. A 340/360 4 bbl intake will bolt up onto a 318 with 318 2 bbl heads, but there's a port mis-match. I does seal fine and I've personally run a 318 with 318 2 bbl. heads and a 340/360 4 bbl intake. I've also run a 318 with 360 heads and a 340/360 intake as well.