I bought a professionally rebuilt 350 short block about ten years back, never did finish building it. It's been on an engine stand, oiled fairly regularly and covered with a garbage bag. No visible rust, except a tiny bit on the harmonic balancer that I think should be able to come clean off with a little emory cloth.

Is it still good, or should I assume that it's seized?

 

Sounds like it would be fine.....no rust? Not in the bores? that's what is important.

 

correct, just the light bit on the balancer.

 

Turn the crank, watch for ring marks in the bores. If none, then it should be fine to build.
Because the assembly lube is so old, you'll definitely need to run an oil pump primer mounted in a drill, before starting it. Turn the crank, slowly, 2 full turns, while a helper runs the drill. You'll need to do an oil and filter change after the first 15 minutes.