I am not an expert on the 420, but generally speaking here is how I do it:
1) pull the wire from the coil to the distributer and lay the end of the wire about a half inch from the metal of the engine. Crank the engine. You should see a hot blue color spark jump at least a half inch with a snap. That proves the coil and wire to distributer is ok.
2) Wires usually measure about 7K ohms each. If they measure 20K, there is a problem.
3) Reinsert the wire from coil into distributopr and pull spark wires one by one and repeat the initial test. Lay each wire one by one a half inch from engine block and crank engine. Observe that spark is hot, blue color and with snap. If all wires show weak spark you have bad distributor cap or rotor, or bad wires.

Thats how I do it.
Rich