415HP from 318ci IS POSSABLE!!
The machinist took 5 pounds off a 340 crank and got it balance. The mallory metal alone would cost 500 bucks. I'd say there was 4K+ into just the crank including turning down the rod journals, profiling and re-hardening. That Holley 950HP runs about $800 before any of the custom work was done. Custom Diamond pistons are over a grand, not including the machinists time to measure everything up. I see this build topping 5 digits easy.
Now here's a build that 470 HP at the crank by BJR Racing: http://www.forabodiesonly.com/mopar/showthread.php?t=28974
It could be done at less expense than the aforementioned build but if you PM'd BJR, I imagine that he wouldn't call this a daily street driver. :)
About the only thing that I would change is I would use the 85-89 318 pistons instead of the KB167's as the pistons would be in the hole about .030 or so and this would drop the compression ratio to a pump gas running engine and should yield a 10:5 compression engine or just over. The rest of the engine could be run on the street and the gearing and converter would be changed, along with the carb to a 3310 VS unit. The 112 centerline will make this engine very streetable, and have plenty of low end TQ. and HP. This engine would be good to 6,000 RPM's. This engine would make 219 HP and 384 FT. Lbs TQ @ 3,000 RPM's and 484 FT.Lbs peak @ 4,500 RPM's and 501 HP @ 6,000. So IMO I wouldn't go through all the rod work on the beams but would resize them, as the street wouldn't ever see the performance gain the light weight rod's achieve. But it does make them more durable.