Stan, the interference would be primarily from your copper wire HT leads.
Apologies Stan...I've jumped in probably too quickly here as I just realized it's a 3 litre your referring to. It was the term 'electric points' that threw me! Solid or wire core HT leads are not compatible with radio reception unless you've got radio (old style) suppressors screwed to the end of each HT wire and definitely not compatible with any electronic ignition modules. The best leads to use for both your radio reception and an electronic ignition module if you're fitting one are spiro core LPG leads...not cheap but will provide the best outcome for both the ignition system and the radio, whether points or electronic module.
NB.
As for the coil primary resistance you generally need at least 3-3.5 ohms across the primary circuit of the coil for points. If fitting an electronic ignition module...this would also depend upon any resistance included in the 12v feed to the coil. I believe P5Bs have a resistor wired into the ignition circuit (but this may not apply to your 3 litre) my guess 1.3 ohms...which would drop the voltage down to 9v at the coil. On a P5B there is another feed wire to the coil (wired from the starter solenoid) that provides a full 12v only during cranking...when the starter heavily draws on the battery and there is a voltage drop. This won't hurt the module during the brief period of cranking. I doubt your 3 litre has this 12v shunt to the coil (but stand to be corrected.)
If it does and you're looking to fit an electronic ignition module, add the resistance of the primary ignition circuit 1.3 ohms to a suitable coil (of say 2 ohms primary) to give an overall primary resistance of (1.3 + 2) = 3.3 ohms.
It's the total resistance of both the coil and the primary power feed (from the ignition switch) that you need to be aware of.
* Sometimes finding an appropriate coil can be a pain. I check the primary resistance of any coil I buy at the counter with my multimeter before purchasing. The parts guys generally don't have a clue, they simple work off part numbers and all oil filled tube type ignition coils look the same externally...don't they?!
Hope I didn't confuse you with all the above!
This vid may prove helpful.