My 292 TBird has the usual upgrades, "G" heads, Isky cam, 57 distributor, "D" code 4V Iron Intake, Holley 0-1848 465 cfm vacumn secondary 4 barrel, 4.11 gears and O/D.  I'm thinking about trying a 600 CFM Holley double pumper for better street performance. Is this a better option or am I going backwards?

I would think a 3310 Holley

The Holley 465 is a great carb for street performance especially off idle. What transmission and rear gears do you have? Fun street performance is usually in your gearing and if you have the stock Ford-o-matic a trans starting in first gear will make a ton of difference. It did for me.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

What transmission did you use?

Here’s a link to Ted’s carb testing on one of his motors. You’ll have to draw your own conclusions as to how your engine compares. The 465, 600, and 750 are included among others (16 total).

http://www.eatonbalancing.com/2012/09/25/four-barrel-carburetor-testing-on-the-y/

I’ve never been a big fan of double pumpers on street cars, it takes a lot of fuel to cover the flat spot at low rpm’s. Race cars, different story. But opinions vary on carbs, just like cams and mufflers (and everything else).

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

My car is a three speed manual O/D with a 4.11 gear.  The 465 is a great street carb.  I'm looking for a little more excitement when I hit step on the gas.  It seems I'm never satisfied.   :>

Bama. I had a 1962 T-bird medium case cruise-o-matic Because of the speedo gear location I had to install the Ford-o rear housing which was machined and rebushed for the larger yoke.
 If doing it again I would adapt a C4; direct adapter bellhousings are available 

56 Vic, B'Ville 200 MPH Club Member, So Cal.

I have a 59 292 bored 30 over and mummert intake. headers pertronix and i believe the rear is 3.25 . origial 4-speed with granny gear. I have had several different carbs on my motor and by far the best overall carb is the 500cfm edelbrock AVS2. idle is incredible, no flat spot, starts like fuel injection. I have been extremely happy with it.

That data in the link posted above is over ten years old and is outdated.  Since then, there have been some new players involved when it comes to carburetion for the Ford Y family of engines.  If the engine is a stocker, then a small sized carburetor is fine.  If never going over 5000 rpms, then a small carburetor is also fine.  ‘Small’ being 400-500 cfm in size.  If the camshaft, cylinder heads, and/or exhaust systems have been upgraded, then the engines will simply prefer a larger carburetor to capitalize on the new found range of rpms that will be available.
 
As a general rule for vacuum secondary carburetor selection, always select a carburetor that will be at minimum, oversized for the application.  This has to do with the vacuum secondary side of the carb being opened based on engine demand.  That means the secondaries will only open as much as required and although the carburetor may be as much as 750 cfm or more when fully opened, it’s simply going to be less cfm if the secondaries are not fully opened based on not being needed or in demand.  It's the best of all worlds in this scenario as the larger sized vacuum secondary carburetor will always be the right size regardless of conditions.
 
That brings us to the double pumper carbs that so many people have a love affair with.   By design, they are on the ‘rich’ side out of the box and that’s because they were designed for a racing or high performance application where all the aforementioned performance mods have been made to the engine.  It’s a rare day when a double pumper or mechanical secondary carburetor is the ideal size for a street application as they are typically either too big or too small.  On that rare day when it’s just right, that can change with the weather the very next day.
 
I have installed a multitude of vacuum secondary 750 cfm sized carbs on the Y engines and they drive great while providing sufficient air flow for engines running well in excess of 5000 rpms.  My overall choice for a 750 carb would be the Summit carb which is a remake of the older Holley 4010 series and that was a remake of the older flattop Autolite/Ford carbs used in the late Fifties and Sixties.  These carbs are a modular design not requiring the every three year rebuild a typical Holley carb seems to require.
  
This years Engine Masters Challenge entry was 303 inch Y with iron heads that used a 930 cfm double pumper (mechanical secodaries) and was making 430HP on the JE Pistons dyno.   And that engine combination still managed to surprise everyone by idling at a nice 550 rpms.  This engine was taken to 7200 rpm for each of the twelve pulls it made at the competition.  That kind of performance was only available by using a carburetor sized appropriately for the increased air flow of the engine.

Lorena, Texas (South of Waco)

Great info. My Holley 465 performs well, has great throttle response with no dead spots.  It doesn't look all that cool.  I'll keep my hood closed.