I've never had a 312 but the same principles still apply...
The first thing that I would check is your grounding circuit. Old Ford distributors get their ground from the engine through the tightening clap and bolt. Rust or ant-seize compound can increase electrical resistance.
Use a VOM and check resistance between your battery negative and block, block to distributor. Both should be zero. In some cases I've installed a #10 wire from the battery to the block, manifold, heads to get this down to zero.
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