Does it miss all the time? If not when? hot cold what RPM and in hg does it start to miss at? reluctor gap. rotor phasing. plugs. plug wires. less likely (vac leak/lean jetting). I wouldn't think 70's is in danger of being lean but bumping up 4 sizes for a 1 hour test would be easy to elim that & might partly compensate for a vac leak just for diagnostic purposes.