The general rule is to use 80% to 85% of the R12 charge when filling with R134A. That means around 1.9 to 2.0 lbs of R134A. As far as an orfice tube, your car may have an expansion valve rather than an orifice tube. If it has a valve, it will be mounted on the inlet of the evaporator under the dash. The orifice tube would be mounted in the hose running from the condensor in front of the radiator to the evaporator, possible right at the inlet to the evaporator.