To determine if it is an SS just look at the trim tag. If it is an SS it will start out with the word Style 64-1467 Body. Bear in mind that the SS is body style not a performance option. If you look at Old Reliable Super Stock race car you will note that it was a Biscayne two door sedan with a 409 425 horse and Borg-Warner four speed but little else. It has a bench seat, and little else (not an SS). The SS was available with a six cylinder (if your trim tag has 1367 it left the factory with a six cylinder).
What you are proposing is a fine ambition. If it is going to be a driver for cruising with the family the motor's originality isn't all that important, and a 350 makes more torque than a 327 to offer better acceleration. It can be dressed up to appear closer to a 1964 SBC, but you will have to go back to a Rochester 4GC carb, and manifold, as well as bore a hole in the block to accommodate the air oil separator that was dropped in 1969 to enclose the PCV system in the valve cover (less machine work involved in preparing the block, which saved GM money). Since you don't have the riser cast at the back of the block on a 1963-'68 for the external PCV plumbing you will be introducing an oil leak. But it is the only way to disguise a large journal 1969-'86 4 inch bore block to appear to look like a 1955-'68 small journal 265-283-302-327 block (the 1968 327 as a 350 large journal block with a 307 crankshaft fitted and balanced just for the 327 rotating assembly).
If you look at the changes made every year in the evolution of the small block engine you can tell what year it is just by the changes made from the original 265; though they all look alike from the outside (mostly). You can find a book that lists the interchange parts with pictures in your library to check out exactly what your engine should have on it for 1964.
Hagerty has your car running from $18,00 to 35,000 for a full frame off Concours white glove chalk mark correct restoration
https://www.hagerty.com/apps/valuationtools/1964-chevrolet-impala
I think your evaluation is a bit optimistic at best. If you think a sale of your car will make money it probably will not happen if you put a lot of money into making it look "original".
Big Dave