Firing order

Firing order for the 4.3 is 1-6-5-4-3-2.

Did you remove the distributor or just replace the cap/rotor/wires? That will give me a point to start from.

Okay, in that case you start at the beginning.

Disconnect the battery and remove all the spark plugs. Turn the engine over by hand with your finger over the spark plug hole for number one (I usually use the extension hose for a compression tester as it lets you reach it easier and because of the small port and the one way valve you can hear the compression easier). You need to bring it to TDC on the compression stroke. You will know it because the air compressing in the cylinder will hiss as it tries to push your finger off the hole.

Now look at the damper. You should see the TDC mark near the pointer. Just put it right on the TDC mark.

Make a mark on the distributor body at the base of the number one wire tower. Now remove the cap. What you want to see it the rotor tab pointing at the mark you made, if it is not then you need to pull the distributor and rotate the shaft so that as you lower it in and the rotor turns because of the gearing it stops with it pointing at the mark.

This is a small block, which is the father of your engine, and shows the same process.
https://www.youtube.com/watch?v=O4WRL513Gx4

Skip the part about the valves because you likely already have the covers on. The compression pressure works just as well.

Once you have it installed you can install the plugs and wire it up and see if it fires up.