I personally Like Quickfuel Carbs & components the best for Holley type carbs
IMO If the heads are going to be ported, you need a little bigger carb,
then I personally would go with the HP 950 Holley
any of the Holley style of carb form Quickfuel, AEM, Proform,
even a Demon {owned by Holley Now}, Holley HP series, Promaxx
none of them are cheap but well worth the extra $$$
Id also highly suggest to get a couple books or dvds,
to learn or "actually" know how to properly size & tune a Holley style carb too
I'd also suggest getting a few different sizes up & down
of accelerator pump discharge nozzles
assorted colors acc. pump cams
assorted sizes of main jets
assorted power-valves
{usually need about 1/2 the # of inches of vacuum, the engine makes at idle}
Assorted gaskets for metering blocks, fuel bowls & secondary plates if vacuum
I would also add that if you get a regular Holley 4bbl,
to get better metering blocks, adj. emulsion tubes
& air bleeds will help to fine tune your specific engines needs...
If it's a vacuum sec. carbs style carb {great for a street driven automatic}
I'd suggest to get a kit/set of assorted diaphragm springs,
so to be able to change rate of opening...
I'd also suggest if it's a vacuum sec. carb, getting adj. secondary plates,
to use reg. Holley jets, from someone like Quickfuel or Promaxx etc.
IMO If your not having the heads ported the HP 850cfm Holley will probably be fine,
depend really what it is you want, what the end use will be, what are your expectations,
any Holley style aftermarket supplier, would be a great choice, even then,
it should still be fine tuned after installation, for your specific combo...
Don't expect to have it be perfect out of the box, regardless who you get it from...
CI x RPM dived by 3456 = CFM
Example;
496ci X 6000rpm/3456 = 861cfm
496ci X 7000rpm/3456 = 1004cfm
that formula is for about an 80% efficient set up,
which is pretty good for non ported heads,
for a race motor or spinning higher RPMs &
getting the most flow capable out of it & ported heads a little more carb is probably better...
It depend really how it's going to be used ?
what rpm's you run ?
what type of trans &/or converter ?
what camshaft ?
what altitudes ?
what gears ?
what head work & cfm flow rate ?
what weight of car ?
there's allot of other factors to consider