I have installed electronic ignition from Pertronix and love it. Flamethrower 3 Ohm coil too.
They recommend removing any ballast resistors as it affects spark performance. Also I have noticed the ageing Smith's rev counter to be slow and under read now(checked with my strobe).
Question : will removing the ballast resistor put anything else at risk (such as the rev counter) in that signal chain?
I know it is there to "help" the coil get enough power when cranking in colder weather.
Thanks.

Yes, the ballast resistor should be removed, and no harm to anything. Resistor was covering for inherent weakness in coil/points setup, resolved with Pertronix distributor. They do recommend "Relay Setup" to get full battery power to + side of coil. Pertronix has their own relay kit for $35, or you can wire your own with 30-amp relay from any auto parts store. You can mount relay on Ignition Coil Bracket. Route and secure #10 Red wire with 30 amp inline fuse from Battery + post, to Relay with tie-wraps as needed.

Several issues with Tach: some models have it coming from + side of coil and late models jaguars having it coming from - side of coil. If yours was coming from + side of coil thru the ignition ballast resistor, then a resistor may be needed between + side of coil and Tach. See attached PDFs for (+ or -) tach hook-up fixes.
Rgds
David


Typical Ignition Relay Wiring

Probably #10 wire to battery post would be fine for the distance you have. Your Tach maybe off + side of coil.