That is a shame. Seems like more and more folks popping up with a shop and don't know jack about anything. If it's not to bad he could have the motor sleeved, but you are talking a little more than he prob wants to spend.
I like the teen myself and with a .40 over taking it to 3.95 bore and putting the money into the top end he would still have a awesome ride. Being the car only came out that year with 273 and 383. I would think being he had the 340 in it, the value prob wouldn't change much. Maybe sell quicker but wouldn't think anymore.