Is this being built as a place holder for the big one? What is your real timeline for the big one? A basic rebuild on a 340 will run around $3000 for a complete long block where I live. If it will be years before the big one and it's driveline are ready, then I would rebuild the 340, stick a modern cam in it, and drive the car. Dont overspend, but it will need to be reliable and able to survive on pump gas.