Seized engine?

[Freddy67]

Im hoping for some good news here. I had a guy replace quite a few parts on a 78 sbc 350. New cam, timing chain, heads, headers, air intake, holley....
It ran OK off an on, multiple small problems, til now.
took it out last night, ran well, then half way home , it started making a squealing noise and losing power. wouldnt take a jump, had to tow it home.
today i saw that the low mount alternator bracket had come loose and was rubbing against the harmonic balancer. I fixed that, charged the battery and it reluctantly started and ran for a few seconds. now it does nothing. Battery tests fine. The starter is one of the few things I havent replaced. I tried to turn the harmonic balancer with a 1/4" drive socket, and it wont turn. Im worried its seized up! any info is appreciated!