The 500 Summit is the only one out of stock. It was when I ordered too and got it a week later. I agree they run out a little earlier in the rpm range but for a street driven engine with less tan 300’s any near 500cfm is great. I’m sure what was purchased will also be fine. There are a lot of y-blocks with Holley 390’s and 465 Avengers made for rock crawlers that are running great. Most over carburate and are disappointed with low end and mileage it’s just the nature of a hot rodder..

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

There is nothing inherently "wrong" with your original Teapot Holley carburetor and Load-A-Matic distributor.   Over 30,000 '55 and '56 Thunderbirds were built with this combination, not to mention all the Y-Block passenger cars. The problems we had with the Teapot back in the day were due largely to aging gaskets or bad rebuild jobs.  They would leak fuel and more than one car would burn a hole in the hood paint or worse.  By the sixties, better carburetors were available, so it was easier to ditch the Teapot for a more modern carburetor. In 1957, the Y-block ignition system was substantially improved with the addition of a mechanical advance. Back then, it was common place to buy a whole1955 or 56 Ford V8 car for $100-$200, replace the Load-A-Matic with a '57 distributor, intake manifold and carburetor turning a formerly mild passenger car into a front of the high school tire burner.   I can appreciate those who wish to keep their car as Ford built it. However, swapping out your Load-A-Matic, Teapot and small manifold with an electronic ignition distributor, "B" manifold and new carburetor would be a huge improvement in performance and drivability.

Early Thunderbird hood clearance is an issue.  I have a 312 "B" manifold on my 292 with a Holley 465 and a Casco 1/2" PCV spacer plate in my 55 Tbird and it clears the hood.  I use an aftermarket air cleaner. I had three early Holley 2 barrels on an Edelbrock intake on my engine for a few months.  I had to run screens in the carbs as the bonnet style air cleaners would not fit.

If you are considering using carburetor adapters/carb spacers, you might want to check hood clearance. 55-57 Tbird hood clearance is tight. There is no room for adapters/carb spacers, unless you use a drop-base air cleaner. 

I have a Summit 600cfm on my 312 B intake and love it.

1955 312 T-Bird Warwick, NY

I have a 55 Tbird with 292, loadamatic and stock ECJ 9510 carb.  I could not want more in performance from that setup.  The original carb is not easy to rebuild properly which is why most shops want to put in a modern carb.  They don't have the expertise nor want to spend the time doing it.  Holley married the carb and distributor for performance.  I hesitate to re-engineer what they did as I don't have their knowledge nor laboratory equipment.   Its tempting to best the engineers of 70 years ago but I try to keep my ego under control.   A rebuilt Holley should cost about $350 pus core and the original air cleaner fits.

If out of stock, then inquire as to when they are expected to be back in stock.  Definitely good carburetors.

I have the Summit 500cfm 4V carburetor on my ’55 Customline with its 272 on the ECZ-B manifold.  While slightly under carbureted, it’s an all around good performer and gets very good fuel mileage.  That line of carburetors is available as 500, 600, and 750cfm.  For any performance minded Ford Y, I’ll opt for the 750cfm version.
  

Lorena, Texas (South of Waco)

The Summit 500 is out of stock. I went with a Quick Fuel 450.

Thanks again for all of your help.

The Summit 500 sounds like the ticket! is this the one?

https://www.summitracing.com/parts/sum-m08500vs

Thank you!!

I’ve used small base Carter WCFB’s from both 1953 Cadillacs and Oldsmobiles on an A manifold along with a 57 up distributor with both points and a Pertronix II. The Cadillac didn’t need a new fuel line since it came in at the back like the Ford Holley. Only needed to make a metal bracket I screwed to the carb stock linkage to use the Ford foot feed linkage. They worked perfect. 

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

For $33 I'd try that adapter and if it didn't work do what I did.  As for carb recommendations, again, based on what I did with my Club Sedan, I'd get another Edelbrock.  Mine was the 1404 - 500 CFM with manual choke.  It really woke up my old 272 from it's 2 bbl slumber.  I never had to mess with it.  Worked great out of the box.  
The only reason I snagged a Holley for my current Y block is because that's what I took off of it- so the choke cable, throttle linkage, etc. will all work without having to mess with it.