What driver does a 6.3 A NEMA 34 stepper need?

Use a bipolar stepper driver rated above 6.3 A per phase, with a supply voltage matched to the motor's rating. Undersized drivers will current-limit or overheat.

Is this a drop-in replacement for a smaller NEMA 23 motor?

No. NEMA 34 uses a larger 86 mm frame and different bolt spacing, so you need a matching mount and coupling. The higher current also means a new driver and supply.

Does it have an encoder or closed-loop control?

No. It is an open-loop stepper, so it can lose steps if overloaded. For position feedback you would add an encoder or choose a closed-loop motor.

How much holding torque does it produce?

Torque depends on the winding and driver settings, and no figure is stated here. Check the motor's datasheet for holding torque and inductance before sizing your drive.

How should it be mounted and cooled?

Bolt it to a rigid metal mount that also acts as a heatsink, and leave airflow around the body. Running at 6.3 A continuously makes heat management important.