Spend what you can. My choice for an SC, in order of preferance: 1. FE, 2. 460, 3. 427W.
As for being streetable, my 427 SO is fairly stock, 10.5 compression, with a moderately agressive cam, and is fine on the street. I use 91 octane + additive because I can't get 93 octane in California. I drive the car all the time (three times to work this week). The only real downside (and I don't care about gas milage much) is getting the motor warmed up on chilly mornings (below 45 degrees). I have to keep the revs down for about 15 minutes to get the
oil temp up. Not really a problem because a BB can idle around town in any gear without a hhitch. Tire spin is controlable in anything but the wet, but I don't drive in the rain anyway.
So, let your wallet be your guide. Big cubes is better, FE is best.