If your goal is only 300whp then the stock block is fine. It will probably be cheaper to find a case, crank, rods, pistons, and bearings and do it all yourself though.
The cheapest I've ever heard of a new sb is $1,500, but I think that guy had a hookup. I've always seen them go for $2k.
If you build it yourself:
Good case that doesn't need any work: $450
Pistons: New OEM probably $250, new forged $5-600
Gasket kit: $240
Depending on forged or oem pistons and bearing cost you could save at least a couple hundred bucks... I'm guessing high on the crank and case as well. You can probably find them for less than $700.