There are two different "valves" of which you may be referring:
(1) The vacuum motor, a metal cannister maybe 3 inches in diameter housing the diaphragm which opens the end carbs.
(2) The secondary lockout water valve, a "pot metal" casting inserted into the water jacket in the front of the manifold, and retained by three small screws.
Both are available, neither are "for cheap".
If you are referring to (1), you might wish to consider the mechanical linkage.
If you are referring to (2), simply bypass the valve by reconnecting the vacuum hoses. Most of them are no longer functional anyway; nor you would not have the secondary lockout on mechanical linkage. Just remember to NOT exercise the end carbs until the engine is at normal temperature.
And Jack (post above) is right on concerning driveability. When I "upgraded" to mechanical linkage on my '64 GTO with the two-speed dogmatic transmission, I increased (rather than decreased) the 1/4 mile times! Perhaps the Pontiac engineers really knew what they were doing when they continued to use vacuum linkage with automatic trannys after mechanical was available with the sticks.
Jon.
__________________
"Good carburetion is fuelish hot air". 
"The most expensive carburetor is the wrong one given to you by your neighbor".
If you truly believe that "one size fits all" try walking a mile in your spouse's shoes!
Owner of The Carburetor Shop, LLC (of Missouri).
Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.
Last edited by carbking; 09-24-2015 at 08:16 AM.
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