The four throttle bore holes in this "B" manifold will fit a Holley 390 cfm carb directly - but if you use a 600 cfm - you will either have to have the deck milled flat and the four holes bored out to match the bigger carb - or use a phenolic spacer / adapter to space it up a bit and smooth the air entry into the smaller holes.

You can also get an adapter/spacer from Speedway to fit on your "A" manifold - I don't have the part number at hand - but its a decent way to get the newer style carb on your old style manifold (without big bucks).  

Do you have the teapot fixed except for the off idle stumble?  Maybe a swap to a later distributor will help that problem.  You're going to swap it anyway when you swap carbs, so maybe try that first.

John

"In general" the Holley 4150 carbs have both a primary and a secondary metering block with removable jets (sandwiched between the fuel bowls and the main body) - and the 4160 is a "cheaper" version of the carb that uses a "metering plate" for the secondary fuel control rather than a "metering block".

The cheapest easist simplest one to find is the "1850" vacuum secondary version (this "list number will be stamped on the front of the choke housing). It is rated at 600 cfm - a bit big for our applications - but not too big so long as the vacuum secondaries are kept under control.

You're right in that the early setups are capable of very good fuel economy.  Those original setups with the Load-O-Matic distributor in place are analagous to the speed density system Ford used for fuel injection control in the late Eighties before going to mass air.  If any changes were made to the engine (ie. camshaft change) that affected the vacuum signal, then the engine just simply did not run right; Load-O-Matic and speed density systems are both similar in this regard.  The late model carb and distributor setup is similar to the ‘mass air’ change and simply makes any modifications performed to the engine either transparent or easily adjusted for in regards to the tuneup.

As a general rule, the Y’s like a lot of ignition advance for the best fuel economy.

         Only on the ThunderBirds. The car damper has the marks on the part of the damper that's not rubber mounted. Doesn't change.

   Larger carbs are not intrinsically bad for mileage. There were several different versions of the Holley 600, with varying primary sizes. If you pick out a Carb with small primaries you will be able to generate respectable mileage numbers. The Secondaries will not operate unless the throttle is floored and manifold vacuum rises to a point that demands more airflow. They don't operate very often on the street. 

     Probably the biggest issue with the loadamatic is that nobody understands them anymore, and most of them no longer function as they were intended. Also most of us have issues with the Teapots and todays gas. 

Not to belabor a point, but you have the damper info posted backwards.  The passenger car marks are on the rubber mounted damper ring, the T-Bird marks are on the fixed, non-movable part of the pulley.  Don't want everyone to assume their passenger damper can't slip.

John in Selma, IN

As Daniel mentions, the damper timing marks needs to be correct.  But with that checking out, then the condition of the engine has much to do with how much more initial advance the older Load-O-Matics can handle.  Simply said, the more worn the engine, the more initial advance that can be tolerated without detonation.  This has to do with the vacuum signal being reduced as the engine wears and the advancing of the timing simply compensating for this.  I visualize as much as 12° intial on a well worn engine where the vacuum signal is greatly reduced.  If the timing chain is stretched, then the vacuum signal is also reduced which in turn would allow more igntion timing.  Experimentation is the best solution  but I’ve found that the factory settings typically give the best results on engines that are in good condition.

Installed a maodified 1/2" phenolic spacer to funnel the bigger carb throats into the smaller bores of the Holley base manifold.

The 312 seems to like it and the only changes I've had to make are installing a set of steel needle and seat assemblies (.130) - and a stiffer vacuum secondary spring (the Holley black one). 

I believe she will make a good bit more top-end power if I take my spare manifold to the machine shop and (1) have the carb deck milled flat and (2) slot the two fore/aft throttle bores together side for side (so its like the Blue Thunder manifold). Then use a larger phenolic spacer that matches the Demon throttle diameters.

The all vacuum '56 distributor will not work properly with any but a '56 carb.  The '57 and later distributors have both vacuum and mechanical advance, work much better.  Actually, your '56 will run better with a later distributor and your '56 carb, and it isn't an expensive change.  You'll need the later cap and rotor also.

John in Selma, IN

TC