800hp from 318 cubes is possible. On 15lbs, custom rod & piston combo, good quality heads, custom cam (for your exact combo) turning it 7500-8200rpm, modified oiling system, studded everything, maybe even a main cap girdle. Balanced, blueprinted, damn good heads, and an excellent ecm system, assembled and tuned by a pro.
With that goal, you're gonna be in this 15k easy before you even start on the forced induction system. you would actually save money if you would go and get a 360 block and a complete bolt-in forged 4" stroke crank kit. Those 80 cubes will save you the costs associated with components required for the high rpm duty, they will broaden your power band, and even reduce your boost requirements. As well as giving your build room to grow later on.
If you build this 800hp 318, and lined up against someone else running a boosted 800hp 400 small block in a similar car, you'll lose 18 times out of 19 just by virtue of the broader power band from the bigger motor, and they'll have less money tied up in their combo.
in terms of street power, in the 300-500 hp range, it's entirely reasonable to build anything from 273-400 cubes. But at the power levels you're talking about, things get expensive in a hurry. Much Much moreso for the smaller engines than the bigger ones though. Hell, above 600hp I'd argue it's more cost effective to buy and build a late model hemi than a LA or Magnum smallblock.
As far as the crank, you're not going to break a crank. Talking with local racers, something else in the motor is going to break first, and that in turn might break the crank, but your crankshaft isn't going to be a weak link unless there's a defect in it.
