My dad rebuilt the 350 in his 89 GMC and was having a similar issue with getting it to crank. He assured me that he had the plug wires correct. Not so much.
It just needs compression, fuel, and spark to run. Here is what I would do. Triple check the plug wires. That's the easiest thing right off the bat. Then check each plug to see if you are getting a spark. If you don't find anything wrong, try to start it by spraying some starting fluid into the TB. If it still won't start, pull the valve covers and have somebody turn the motor over while you observe the action on all the valves. You probably need to adjust them all again after monkeying around with things anyway. Once you check all these things, my money is still on the distributor being stabbed in 180 degrees off.
It's going to be something simple when you find it.