Here are a couple of pictures showing the venturi assisted vacuum circuit for the LOM distributors in a ’56 Mercury WCFB carb.  That section of the circuit will need to be disabled if wanting to use the vacuum advance on the ’57 and later Y distributors.  Blocking the indicated port in picture #2 could be done by threading the hole either 8-32 or 10-32 and using a set screw.

If the WCFB carbs are off a 50’s era GM product (Cadillac, Olds, Buick), then the staged or ported vacuum signal you need is already there. Hooking up the distributor to only one carb (usually the rear one) will give you the necessary vacuum signal for the ’57 and up Y distributors.  The external staged vacuum port on those carbs will be located between the two idle mixture screws.  Those carbs also had mechanical secondaries.
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If the WCFB carbs in question have a vacuum pod on each to work the secondary throttles as well as a spark advance valve located between the idle mixture screws, then that is a Mercury WCFB 4V carb which was originally designed to work with the ’56 and earlier Load-O-Matic distributors.  The distributor port on that particular carb is located above the spark advance valve but that circuit also incorporates a venturi assisted vacuum signal which makes it unsuitable for use with the vacuum chamber on a ’57 and up Y distributor.  While I’m sure that the venturi assisted part of that vacuum signal can be blocked so that only the staged signal at the primary throttle blades is the only vacuum source for the distributor, I haven’t had to do that yet simply due to the small number of those carbs that are still around.  When running a pair of those carbs on a dual quad setup on the dyno, it was done using only the mechanical advance portion of a MSD distributor.
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I did look at the bottom of one of those Mercury carbs sitting here but it will require some disassembly to see exactly what modifications would be required to make that carburetor compatible with the later model Y distributors. Without the modifications similar to what has been done to the Teapot 4Vcarbs, I would be worried about too much vacuum being supplied to the later model distributors when driving with the throttle open any more than just for cruising.  Short term, a properly curved distributor with only mechanical advance capability would still give good overall performance without having to hook up the vacuum advance portion of the distributor.

What was the original application for the WCFB carbs you are using?