Your 396 left the factory with a 780 cfm Rochester QuadraJet if it was rated at 325 horsepower, a Holley 650 cfm 4160 vacuum secondary if it was rated at 350 horsepower and a 780 cfm 4150 series Holey vacuum secondary if the 396 was the special high performance 375 horsepower 396. Because only the big full size cars and the Corvette where allowed to have an engine bigger than the magic 400 cube limit imposed by GM's executive board most people who ordered a big block in their Biscayne or Impala ordered the 427 back as soon as it became available as a RPO in 1967. (Regular Production Order).
In 1966 the only way to get a 427 was to build one yourself out of the special production HD service part catalog by ordering the bigger bore block, crank shaft (which differs from the 396 crankshaft as it has heavier counter weights to balance the bigger pistons), and a set of those TRW 12.5 :1 pop up pistons. The open chambered head didn't become available as a service part until 1969 so you were stuck with a bathtub 101 cc combustion chamber.
That being said from 1967 and up few ordered the 396 in the big b-body with the 427 being offered for about the same price. Those who did order the 396 chose the 360 horse version with the Holley 650 vacuum secondary 4160 series carb (no rear main metering block) and a slightly hotter cam than the 325 horse version that wasn't offered in the Impala (only in the Camaro and the Chevelle).
They purchased this engine over the base 275 horse 327 for it's added size and rugged dependability (it was designed to be a high mileage truck engine) and the fact that you cold get a three speed automatic with the big block instead of the aging PowerSlide. I do not know if the 375 horse was offered as an RPO in the Impala as solid lifters conflicted with the Impala's luxury image (no A/C available with a solid lifter car) and that same three speed transmission wouldn't respond well to a the cam in the 375 horse as loose converters hadn't been invented yet. So as with the Z/28 it would require a manual transmission if you did order the 375 horse motor.
So the trend here is that a 750 is a good choice size wise for a 396, and it should definitely have a vacuum secondary. It is the size I chose to run on top of my 406 SBC in my 1989 Caprice. I was running a 3.73 rear gear with a 700R4 that I had built to the max after it's third time in the repair shop behind a big block ZZ502 motor. By the time I had rebuilt it for the third time it had all of the goodies that the aftermarket offered to make it stronger better than before. The supper low first gear, the 3.73 and the fact that a mid-eighties B-body had been down sized and lightened to be about the same size and weight as a late sixties Chevelle, it really could scoot with the 512 horse 406 I had built for it.