Just noticed i had a similar situation involving heater core. What i tend to do is add .2 per year of vehicle for most of my repairs. in the case of a 1996 it would add approx 4 hrs to labour guide. gives room for rust,siezed componants etc.. seems to work well. I try to monitor repairs like that closely so if i am in it at 5 hrs and know i will be over,i consult the customer. most people understand as long as they are kept in the loop so to say as what is going on..