Seized 400ci small block?

[Anorak]

So I am almost done with my engine swap in my 1983 Chevy K10 that used to have a I6. I got it all put in the truck and wired up, when I turned the key it started right up, but the timing was a little off so I shut it off to adjust the distributor. After that I started it again, and it ran for maybe 20 seconds before I shut it off again. I decided to shim the started a little but after I did that the starter didn't turn the engine. So I took the starter off and tried to just turn it over by the flywheel and that didn't work either. So then I put a socket on the crank and it wouldn't budge. I didn't run it for very long, I think less than 45 seconds and I had 5 minute breaks in between, and there was oil in it. I don't understand what it could have been. I talked to a few people but they didn't give me much to go on. One person said that it could be the timing chain broke and wrapped itself around the crank but I cracked the top of the timing chain cover and looked down and I didn't see any issues. I drained the oil and there was a tiny bit of water but not enough that I thought it would be an issue. I didn't see any flakes of metal in the oil. Any ideas or help will be appreciated.

[Gmc.guy]

Maybe try pulling the plugs and see if they are clear. You could have a coolant leak internally and hydro locked the motor. Or a stray bolt inside a cylinder.