Depending on the mileage on the motor, you should rebuild all 8 cylinders & heads at the same time. If you only do a certain amount of cylinders in a short amount of time you'll be repairing the others.
Depending on what the problem is, you would be looking at around $2500.00 for a complete rebuild. I don't know of any shops that would only repair 2 of 8 cylinders.
