Need help squealing/grinding noise

[72again4x4]

5.3 mated to th350 transmission, has the spacer and dished flex plate. Transmission is used, got it off of Craigslist with unknown miles. PO said ran and shifted fine. I did drain 2 quarts out of the transmission when I got it due to fluid coming from the dipstick hole. I never separated the torque converter from the transmission. The entire unit came to me in original used condition mated together 350/350/205.
I’ve taken my time getting everything together for the LS swap and my hope is that it’s just the spacer squealing on the torque converter or something simple. Am I thinking of that correctly or would those two pieces move together?
Upon inspection, there is nothing visible between the flex plate, torque converter and back of the engine.
The starter easily turns over the engine with the gear returning to the housing and not hanging up. The flex plate doesn’t come into contact with the block or any other mounting surfaces.
Sorry, I’m really at a loss and any ideas/help are greatly appreciated.