'72 - Plymouth - Valiant - 198 - Straight Six - tough to start, and rough idle
Is it safe to bypass the ballast resistor just to try to get it running.
It's ok for testing. Internet troubleshooting is mostly guessing.
You say the distributor has been replace, 72 should be points type. Recheck gap.
The rain(may) have something to do to with your problem. Not likely but possible.Dry everything, cap,inside distributor with compressed air, check all wires.
Weak spark, Check for a good zap spark at the coil wire from cap to engine block.Could be the coil is weak.
Other thing Slants are known for: Check the carburetor for loos bolts and screws.
This is way out there but intake manifold nuts loose or blown gasket.I've seen it on Slants. A good back fire can cause a world of hurt.Fuel filter checked ?
Check for any vacuum leaks. That's about it without being with the car.