interestingly, I have heard of this issue several times in the past however I don't see any reference to it in the brother's ad or their videos..
67-72 TACHOMETER CONVERSION SET - 5,000 RPM
this video does reference the 5k or 8k rpm range option for 'high performance' engines approx 0:30..
from that I would infer that electronic ignition would likely be used..
this tach / clock conversion kit doesn't reference the issue either..
67-72 TACHOMETER / CLOCK CONVERSION SET - 5,000 RPM
so after more searching, couple of older threads popped up..
Repro Tach and HEI
Tachometer Conversion Kit
etc..
good luck y'all!