I have a 78 440 overbore 0.055 It make 700 hp aprox.
4 years in this tune. You dont want thin walls but I dont think you need the big thinkness that some say you need.
Some said the same about Al blocks they move around to much you lose to much HP. If you build one on kill you might need that thickness otherwise its not a issue.
Jim
ps I would give the shop a little break on this He might be trying to do the right thing by you and not screwing He might not be a idiot just has a different opinion.