It's good business to give a close estimate but always stipulate to customer unforeseen problems such as broken intake or manifold bolts and ect. I always tell my customers " It should take x amount of hrs with no snags but there is always possibility of some problem when breaking down and reassembly But as a start we will plan on x hrs."

As for labor on this particular job with no snags, i say 6 hrs if the wife lets me get a good nights rest the day before.