Is this the ported vacuum switch that screws into the P/S head? One of theses should work. Look in RockAuto under "1978 PONTIAC FIREBIRD 6.6L 400cid V8 : Electrical-Switch & Relay : Ported Vacuum Switch" You need the one that is normally closed. I'm not sure of the actual part#.
1978 PONTIAC FIREBIRD 6.6L 400cid V8 : Electrical-Switch & Relay : Ported Vacuum Switch
STANDARD MOTOR PRODUCTS Part # PVS31 Click for more information about this part {Click Info Link for Alternate/OEM Part Numbers} Thread Size - NPT: 1/2" - 14 Thread; 2 Ports;Number of Ports: 2;Number of Hose Connectors: 2; Contents: Ported Vacuum Switch
Black; 2 Ports
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$11.13
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FOUR SEASONS Part # H2943
Ported Vacuum Switch w/3/8" PIPE CONTROL EGR VALVE CONTROL; Formula [Wholesaler Closeout - Private Label Pkg. - 30 Day Warranty] (Only 1 Remaining)
$15.98
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STANDARD MOTOR PRODUCTS Part # PVS79 Click for more information about this part {#21217, 3031518} 2 Ports;Number of Ports: 2;Number of Hose Connectors: 2; Contents: Ported Vacuum Switch
Black; 2 Ports
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$17.86
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STANDARD MOTOR PRODUCTS Part # PVS7 Click for more information about this part {Click Info Link for Alternate/OEM Part Numbers} Thread Size - NPT: 3/8" - 18 Thread; 2 Ports;Number of Ports: 2;Number of Hose Connectors: 2; Contents: Ported Vacuum Switch
Green; 2 Ports
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$18.61
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$19.03
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AIRTEX / WELLS Part # 7P1041 Click for more information about this part {Click Info Link for Alternate/OEM Part Numbers} One of our most popular parts
(Black & White -- 3 Port)
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$26.79
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AIRTEX / WELLS Part # 7P1023 Click for more information about this part {#3033795, 3039491, E936}