You would have done yourself and all the previous respondents a favor by putting
the year (1964) in the "YEAR" position on the header.
You would also be doing yourself a favor if you get a dual-downpipe manifold and
matching front exhaust pipe. These are OEM items and normally last a long time.
They will improve your horsepower noticeably, other things being more or less equal.

If you get the late-model (fuel injection) exhaust manifold you will be faced with
flange thickness differences, which can be dealt with using either half-washers or
spot-facing the exhaust manifold to the same thickness as the mating intake manifold.
This is an easy job for a machine shop.
--
George Downs, Bartlesville, Heart of the USA!