I have a 70 crank and water pump pulley. It came from a 70 400 with AC. Can you provide some measurements and I'll see how this compares.
For the water pump to turn at the right RPM you may need to find out the correct part number for a 70 350 and make sure the crank pulley and water pump pulley is the correct ones. That way you do not have any cooling issures from incorrect sizes.