With all stock 70's components, it would be very difficult to have clean radio reception. The main culprits are running the engine points current through the tach, the mechanical voltage regulators and total lack of shielding in the engine bay.
I did add the "factory" suggested screen in the rear bonnet with only minimal results. It took the liberal addition of filter capacitors, ferrite beads and additional grounds to get a working radio when my 70's S2 was a daily driver in the 80's. I discovered that improving the grounds also improved my ignition performance and tach stability.
In my modern build (not concours) I am going to a pointless ignition, modern alternator with internal regulation, solid state instrument 10V regulator, grounded copper screen in the firewall, engine bonnet plus a few more tweaks to get the radio noise under control.
A sweet sounding engine is very nice but so is some sweet sounding music.
Ron