1968 340 timing

No.... driver side first plug.... 1357. Passenger side 2468. The distributor can only slide in one or two ways correct? Not like a Chevy having to line up the splines.
Correct,
but neither is correct; there is no correct; as to how it slides in, and there is no wrong. Unless you want to be factory anal about it, and if your D is not a Mopar, It still might not be factory correct.
When the rotor and the tower and the plugwire to #1 plug ,are all three aligned, when the MSD sends the spark, then it is correct .
If it runs at all, then it is correct.
Whether the ignition timing is or is not correct, remains to be seen.