I'm petty sure that the issue with your tacho will not be caused by the coil.
BTW the correct terminology is 'transformer' style coil, not 'transistor' style.
The newer style Bosch transformer style coil is a genuine supersession & is electrically identical to the old barrel style (oil-filled) HEI item.
As long as you are using the correct Bosch part no. HEC-716 & you have a full 12V supply to the coil & your tacho is not operating correctly, your tacho maybe the culprit.
An alternative to the HEI coil is to use Bosch no. MEC718 'medium energy' coil which is the item used on the 304 EFI V8s. You can use the smaller diameter distributor cap & 'normal' ignition leads with female HT fittings at the coil & cap posts. You still need a full 12V, but you also need to drop your spark plug gaps to 1.1mm, not the larger 1.5mm used with the HEI system.
DO NOT use the GT40 coil, these are only suitable for Grey motor Holdens, Minis & other old style set-ups which have contact points & a full 12V, with no ballast resistor in the harness. The same applies to the GT40R, which is for points style distributors, but those with a ballast resistor.
Dr Terry
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