I wonder if offset Woodruff keys were used with the old chain to advance the timing as the chain stretched, and not replaced with straight ones (no offset) when the chain was removed? That could be one source of over-advanced timing. Wonder if your records disclose anything, if you have them? One way to tell is to pull off the toothed wheel on the camshaft next time you have the valve cover(s) off - pix on my site.
It doesn't sound like the new chain missed a tooth - each tooth is 18 degrees and you would porbably have valves coming into contact with pistons if that happened.
Timing really s/b correct with a new chain.