One thing else that is different for 64 until mid 66 or 67, is that the lower spring BOLTS to a "flat pad" or PERCH on the rear end.
Therefore the lower end of the "early" year springs differs from the 67 and up style which is a "pig-tail" design which fits down "around" and is retained by a circular shaped spring pad.
Although I have not done it, you probably could use the newer style springs from the '71, under the '64. You might have to experiment with different spring rates to get the '64 at the right height.
Good luck.