LA small block timing is suddenly way off

Because you have no idea where the timing was ever set,, I wouldn't take the missing as a result of the timing,, I'd be pulling the plugs looking for a fouled plug, or bad/burned plug wire,, then once you find the miss,, set the timing properly,, and you will prob have way more power than when you "guessed"....

Re-read your post,, and timing chain jump is a possibility,, did you have the dizzy out, or change/install MSD dizzy, when you did the original timing by ear..?

hope it helps