I read an articel a couple of days ago on a chap who tunes Suzuki motorbikes. He gets 390 HP (apparently dynoed) out of a 1400cc four cylinder Boxer-Engine. But that's turning 10.500 RPMs. It's all an RPM-thing. I just don't see the point in paying a lot of money for a small engine that has to turn so high. You'll get problems with bottom end-parts breaking and the dizzy will run out of spark and you'll get real friction problems if you go far over 6.500 very often on a 305. There's so much more you'll have to do to it to make it work really well... Get a Stroker if you want power at low engine speeds. Building a 383 with 400 horses isn't a that big deal... If you go for a Pontiac, they have quite a lot of torque from the start, drop in a 6.6l Pontiac and give it a nice intake, headers, a hot cam and some decent heads (probably no need to go aftermarket, just common sense porting should do it) and you'll be heading at that number for under 2k.
just my penny worth,
greetings from across the pond

and BTW: what sense does it make to state what length pushrods you need? depends on your gasket, on whether the block is decked, the heads milled, what rockers you use, what valves...