I have a newly built 347 i just installed into my 1993 mustang, new cam (b303), lots of other things thrown in as well, got everything in line, got it running for about 10 minutes, idling a little rough while it was cold which the old 5.0 i took out did as well so i figured maybe just an IAC issue, it sounded great but stalled in my driveway a couple times while idling but would fire up again just fine. One time it died and we just left it off for the night, then the next day i started it again, ran for a few seconds, sounded healthy but died (i normally have to hold the rpm a little higher until its warm, figured thats all it was that killed it). I went to refire it and just heard a single loud clank at the turn of the key, thought it was a starter issue so i got a new high torque mini starter installed, battery fully charged, etc. Went to start it again, same clank. When i checked, the engines not turning over by hand at all, and the clank was the starter slapping against the flywheel. This is really concerning because the engine had oil pressure, stayed at a good temperature, and was just rebuilt before it went in the car. I pulled all 8 spark plugs and loosened all the rocker arms pretty good to see if it was a valve clearance issue or if it would turn with less compression in the cylinders, but still no luck. No warning signs, no noises while running, nothing. Ran beautifully and then died and wont spin. Any suggestions? we kept the motor covered so i cant imagine anything got inside the engine, is there anything i could try or check out? ive been told to put the car in 5th gear and push it but havent had anyone to help me try that yet. any help is greatly appreciated