As far as I know they can build the whole motor for you. Before you talk to them, just have in mind what you want. What quality level internals you need, what kind of mileage do you want to get from the motor, compression, what heads you intend to run, etc.

If your goal is a regular 302 with no performance upgrades, call NAPA or Autozone and price a longblock. You will get a mileage warranty that way, and it will save you some legwork.