Terms like "rear most" and such actually become meaningless unless you are present to point a finger at the "rear" you are referring to (rear of truck or rear of engine".
Engine terminology uses the crankshaft ends to define "front & rear".
The flywheel location identifies the "rear" of the engine.
Belt pulley/vibration damper identifies the "front" of the engine.
Cylinder number 1 is the cylinder nearest to the front end of the engine.
If you are only using the pointer on the vibration damper or pulley as the indicator for Top Dead Center......that may be where you are messing up in aiming the distributor rotor.
Four stroke cycle engines must rotate through 720 degrees of the crankshaft (360 degrees for the camshaft) to make 1 complete operating cycle.
The marks on the damper or pulley will line up 2 times in one complete 4 stroke cycle of the engine.
To correctly use the marks on the damper/pulley as an indicator of TDC on the compression stroke you must also confirm that both valves on the timing cylinder (#1) be CLOSED when the damper/pulley marks line up.
If you have omitted to have cylinder #1 positioned with both valves closed when you install the distributor, you may have the ignition firing at 180 degrees "out of time".
Confirming that cylinder #1 is on TDC Compression stroke can be done by placing a thumb or finger tip over the spark plug hole and feeling for the onset of compression pressure while you "bump the engine" with the starter, or turn it manually with a bar. When you feel pressure against your finger, then move the crankshaft in the normal direction of rotation and line up the TDC marks.
Now you can drop the distributor into its hole and point the rotor at the #1 cylinder position on the distributor cap.
You will still need to fine tune the timing with a timing light later when the engine runs.