slant six cranks but won't start

If you got plug wires into different towers the timing will be off. Usually it will pop, bang, etc.

Check for spark: "Rig a SOLID core wire to a test gap, either an over-gapped plug or a store bought or home made tester, or if two people, hold the wire near metal of the engine

Crank engine USING THE KEY (and not by jumpering starter relay)

Spark should be snappy, hot, blue, and at least 3/8 and more like 1/2" long

Troubles:

Bad "condenser" (Capacitor) Modern chinesified parts are not always reliable. System MUST have good condenser to run.

Possible points are not opening (something shorting like improperly configured wire/ condenser terminal) or not closing--misajusted, or dirty and won't conduct To determine, apply meter or test lamp to coil NEG, turn key to "run" and bump engine by jumpering starter relay. Meter/ lamp should light/ indicate voltage with points open, and go out as engine bumps over to a "closed" location.

If you fooled around and had left key on for extended periods while attempting to start, it is possible that you burned points by leaving key on, points closed, excessive current

Another test:

Bump engine to visually close (or open) points
Apply test lamp or meter with key "in run" to confirm points open or closed

Whichever, either manually open them with an insulated tool, or short across them to ground if closed. Either action should result in spark from the coil if:

Coil/ ignition is getting power
If condenser is OK and not shorted to ground
Coil is good