Reinstalling the heater is an easy job provided that the tubes inside the bulkhead weren't removed with the heater. The blower, heater core, box and valve all are external in the engine compartment and can be in place in 15 minutes if you have all the parts ready. Threading the cable to the control can take a little time and effort since it goes through the bulkhead, but nothing terribly difficult. So it all comes down to the water transfer pipes whose replacement can take time, and a helper if possible.