No crank, no start?

Okay, it sounds like power isn't getting through the ignition switch. Grab the test light, connect it to a ground. Pull the crank fuse, Turn the key to start, check both terminals, one should light it if there is power there. I'm betting it doesn't light. Now go to the ignition switch. Find cavity D1 with a yellow wire in it. That is the crank signal. Use the test light and probe that connection while turning the key to crank. If you have power there the light should come on, if it does the wire between that pin and the crank fuse is bad. If no light, find the red wire at terminal D5 and probe it, it should have full time power. If not, then the red wire between the connector and the fuse box is bad. Now because the starter does crank when you touch the crank fuse, we know that the maxi fuse is good as it supplies both, it has to be the wire. However, that wire only triggers the relay so you could just find another hot wire and run a jumper wire to handle it unless you want to dig out the dash harness to trace out that wire to replace it.

I doubt it is the switch, it sounds more like a wiring issue and not a parts problem. Try the testing above to see if the wiring issue is before the switch or after it.

Okay, so you have power on the red wire into the switch. And when you turn to start the yellow wire gets power? Are you using a test light or a meter? If you jump power to the crank fuse, it turns over, correct? But is there power to the crank fuse when you turn the switch to start? If not, then that yellow wire coming into the crank fuse or the terminal under it that has the problem.

Okay, you can ground the fuse and it starts? That shouldn't be possible. There is a problem with the relay circuitry then. The yellow wire should have power on it only ground should not cause the relay to activate. Pull the crank fuse. Test both terminals with the test light connected to ground, you should get nothing. If the starter turns, then we need to go to the starter relay and pull it. With the test light connected to ground there should be one terminal with power. That would be at A9 where the battery power enters.

Okay, that isn't good. Without the key in the start position there should only be a single hot terminal that comes from the battery and when the relay closes it then sends power to the starter. The other two will be the control side of the relay and it seems the ground side has shorted to power. That is why it won't start with the key, with battery voltage on both terminals they cancel each other out and the relay doesn't trigger. So, we need to find the ground wire, it is black and should go from the fuse and relay center out to a ground connection at the right front of the engine block. It is the ground in this diagram. It shares that ground with multiple other items, one of which is the transmission position sensor.
I would find the bolt it connects to, then remove it, now test to see if you have battery voltage on it, if you do go to the items indicated and unplug them and see if one of them is shorted to power first. Like the AC relay, the brake light relay, the O2 sensors, possibly one of those got pinched when the engine was reinstalled.

Steve W is one of our best! Can you please leave us a rating? (Copy entire link)

https://www.google.com/search?q=2carpros&gs_ivs=1#lrd=0x80dcd47364be5d0d:0xba091aa4209f4497,1,,,&tts=0

and

https://www.sitejabber.com/reviews/2carpros.com

Use 2CarPros anytime, we are here to help. Please tell a friend.

You're welcome. I generally like to add a second ground on the GM trucks as well, they used a lot of braided ground straps from the engine to the body and body to the frame. Those rot off and you get some very odd issues. It's a good use for those pre-made battery cables you see in places like Farm and Fleet or Tractor Supply. It's not so bad on the trucks up to about 1994-5 but after that and especially on the ones built after 2004, they fail a lot.