[A1971Blazer]

I'm in the process of swapping an LS1 into my 67 but I'm doing a full resto so the body is off. I have the engine and trans mounted and I made a set up so I could start it up.
When I started it the first time it had to hunt for idle speed for 30-40 seconds before it settled in and idled properly.
When I shut it off, I killed all power to the PCM (no constant 12V input) subsequently, at every start up it would again have to hunt for idle.
A few times I left the 12v connected temporarily, when I did that, it started and idled immediately.
As mentioned above, check to see if the "keep alive" 12v constant input connection is good.
Good luck with it