I would trust the shop manual if it has specs for your specific truck. If the dwell and timing are correct, it still sounds like it's a carburetor issue. Have you disconnected and plugged the vacuum for the brake booster? that can be a large source of vacuum leak.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
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