Bill, you don't need a ground from valve cover to firewall. They were originally installed only if the car was ordered with a radio. No radio, no straps. They were intended to suppress ignition electrical noise.
Engine is grounded by the main negative battery cable. Chassis is grounded by pigtail off negative cable to passenger side front fender. So engine and chassis are tied together at the negative battery post.
I bought a set of those straps years ago just to make it look original but never got around to installing them. Been driving the car for 9 years without them with no electrical issues. If installing them solves a problem then you need to keep looking. They're covering up the fact that you don't have a good ground from the battery to chassis or battery to engine.
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Hank
70 Chevelle SS396(454), M20, 3.73
My Chevelle
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