That’s a loaded question!
First of all, there were there different engine “families” built in the 318 displacement. All used identical bore and stroke, but shared little else.
1957 through 1966 (’67 in Canada) it used heavy polyspherical heads, this version is easily recognized by the 2-bolt valve covers. This was known as the “A” engine.
1967 (’68 in Canada) through 1991 the 318 was an LA engine — literally, lightweight “A”. This used modern wedge combustion chambers.
’92 through ’02, the 318 was again thoroughly modernized – improved heads, manifolding (intake and exhaust) multipoint injection, serpentine belt, and a slew of other upgrades.
Then you run into the options! For instance, the dual four-barrel 318 poly in the ‘57 Fury was rated at 290 HP. The smogged-clogged late-’70s engines were rated at low as 145 net HP (see below). There was also a 318-4BBl cop engine in the ’80s that used 360 heads and manifolding.
Then there’s the measurement system! Starting in 1972, HP was measured as “SAE Net”, which, meant as installed, fan, exhaust system, air cleaner, alternator, and so on. Prior to that, the numbers were gross, which produced much higher advertised numbers.