Compressor gasket problem

I have my 71 Dart at a local shop for AC compressor replacement. They are having trouble getting the high pressure line fitting to seal on the discharge side of the compressor. The fiber gasket furnished with reman compressor didn't seal at all, they tried a metal gasket and got it down to a very slow leak. Both the surface on the compressor and the fitting seem to be flat. Any suggestions for a different type of gasket or a sealant suitable for AC fittings?