Engine will not turn over

Do not remove the starter to have it tested. That is a lot of work for something that is not likely to be the cause of the problem. Doing guesses like that often causes new additional problems with broken wires and twisted off electrical terminals. They often check fine on the bench when there is no load on them. Remove and/or replace parts only after testing and diagnosis proves they are defective.

You know it is not a starter motor problem if you are not hearing the starter relay click. Battery connections are not the problem if the dash lights are working. If the dash lights go out or the head lights get dim or go out when turning the ignition switch to "crank", then suspect battery connections or starter connections. GM likes to tap off the large battery cable at the starter motor to feed other systems on the truck so if the lights dim or go out, Then check the large starter connection.

For a do-it-yourselfer, a quick check of the starter system is to pop the cover off the starter relay, reinstall it that way, then squeeze the movable contact. The system can be broken down into three circuits, the low, medium, and high-current circuits. If the starter cranks the engine when you squeeze the contact, you'll know the medium and high-current circuits are working and it is the low-current circuit with the neutral safety and ignition switches in it that has the problem. If it still does not crank you will know it is a waste of time to investigate that low-current circuit.

When all else fails check the neutral safety switch.

https://www.2carpros.com/articles/how-a-neutral-safety-switch-works

Please let us know if you need anything else to get the problem fixed.

Here are some more diagrams I found (below)

Cheers, Ken

Does it crank ok? If so, have a helper crank it, while you visually check for spark at the plugs.

Check the voltage feed at the ignition switch, red wires, should be hot, gets feed from battery fusible link.

Yeah, I'd start by checking feed to the ignition switch. Maybe you can get somebody to help you drag it home, tow rope, or call a tow-truck. Then you can work on it at your leisure.

Check the battery condition and connections if okay could be one of the following starter, park and neutral switch and ignition switch

Check the fusible links that run off of battery for being burn up see pic

Are you getting fire to the plugs?

Check all connections from the battery to engine to starter-do a voltage drop on the cable

Or could be a tight engine that needs help to crank over such as removing the plugs and spraying WD-40 to loosen it up. I just hope its done right rebuilding it. I've seen 283/307/350/454/351/360/390/460 won't even budge when crank over because the rebuilding process wasn't followed accordingly. Its like the engine seiged or frozen stiff.

Good Luck

Good Luck

Do a battery capacity test-disable the ignition -use the starter as a loading device-hook the meter to the battery terminals-now crank engine for 10secs-while you read the meter-it should read 9.6volts or better within that 10secs if not replace the battery.

Did you put all the shims back? Slow crank can be that or timing too far advanced, bad battery cables.