Hi Aldo,
Welcome to the forum. Nice car!
There are no really good production records, but I can can give you some ideas.
Of total 1965 production of 559,451, 35.6% were 6-cylinders and 64.4% were V-8 - or about 198,900 and 360,600, respectively.
And there were 71,303 63A fastbacks produced. If the ratio of 6-cylinder engines is the same for the fastbacks as it was for entire production (which this might not necessarily be true), then about 25,383 were 6-cylinder fastbacks.