The clearance recommended in the AWS Brazing Handbook, 4th Ed is 0.000 to 0.002 in. press fit.   This assumes that roughness from machining is sufficient to allow the copper brazing filler metal to find its way through the Joint.  An average surface
Roughness of 24 ?in. to 64 ?in. (0.6 ?m to 1.6 ?m) Ra
is generally acceptable.
If this is to be brazed in a vacuum furnace, the vacuum of 10e-3 to -5 torr would be recommended for the two base metals indicated.   Unfortunately the Copper will heavily vaporize and it will condense in the heat shields.  To reduce the copper vaporization it will require a partial pressure in the vacuum furnace by adding dry nitrogen or argon to provide a partial pressure of 1000 or 2000 microns.   This will slow the vaporization of copper but not eliminate it.   If you were to use a nickel base brazing filler metal such as Nicrobraz LM, BNi-2 the partial pressure would not be needed.  Brazing would accept a vacuum atmosphere of 10e-3 to -5 torr.  See the WCC web site.