1965 valiant 225 timing issues.
I need a little help. My daughters 65 valiant is hard to start and shakes.
The firing order on a slanty is 153,624
And the slanty only fires 3 cylinders per revolution.
So in your case, the good cylinders 5 and 3, are dragging that deadbeat #1 around on the first revolution; then on the Second, cylinders 6 and 4 are dragging the even deader #2 around.
Then consider that 1 and 2 are on the front of the crank, and side by side.
The engine has to shake, it doesn't have a choice.
Finally
consider that the headgasket may be blown between those two cylinders. If that was the case, then the crossfeed between them will upset the carburation, as the pressure pulses between them, and up into the intake.
I strongly recommend a Leak-Down test to prove the low pressure is NOT a headgasket.