I'm with norm on this one, your guy doesn't sound like he knows what he's talking about. There's no such thing as a cracked flathead, and the rod issue doesn't matter. I never heard of matching head and block numbers because Mopar never did that on flatheads.
Changing parts is just a way of coveribg yourself when you don't really know what you're doing.
There is an old saying that yoiu shouldn't BS a BSer. I know BS when I hear it because I'm pretty good at it myself. I believe that you have heard it also from a "mechanic".
If I were you, I would gather up my parts and run away fast.
This is all just my opinion, and its free, so take it for what its worth.
I have a dislike for people who take advantage of others ignorance.
To quote King Arthur on his quest for the Holy Grail, "Run away fast". No one wants to get hit by a flying cow.