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Let me preface my remarks by saying that i have never messed with a small block mopar.
I think the 318 reputation suffers from never having a prformance version, unlike the 302 ford and 327 chevy, virtually the same size engines.
I would think that a 318, with the right aftermarket parts, such a cam ,compression, and cyl heads could have comparable output, even though there are many more choices for parts for the 302/327.
I dont think a streetable 420hp is too difficult with the ford or chevy, so i would expect that from a 318 too. Wallac3 says a 420hp engine in a 3200lb a-body (including driver weight) should go 11.50/118. I think that is wildly optimistic for a normal street driven a-body. I would consider a n/a 318 that runs low 12s a darn good runner, even very high elevens would be a killer.
The calculator i use says a 12.50 car at 3300 lbs needs about 340 crank horsepower, not even 1.1 for a 318.