Figure no more than 75% of the original R-12 charge which should be on the compressor tag. On the '66, it was 1-3/4 lbs. or 28 ounces. Be sure to replace the dryer and possibly the hoses that have the high and low pressure fittings. Evacuate for 30 to 45 minutes prior to filling. Converted both of mine to R-134 and it works great. Good Luck.