My best guess is the guy's son did the work and either ran the tap too fast or used a dull one and the heat build up cracked it. I pulled them out but didn't notice the cracks at that time.
Thanks for the recommendations and I'll see if I can get this sorted out and order some parts today.
I'd be very surprised to believe a dull tap caused that much heat to cause cracking or the cause was tapping too fast, but I've been surprised before.
Now, I do not know anyone who rebuilds those mouse motors with that weak link still intact, i.e. pressed in studs. You stated a balance issue but not sure how bad that was and how you found out or the crank was not machined properly, are you referring to the past engine build where you ran it for 4 years or recently they machined it incorrectly. What was wrong with the crank? Now I'm not picking as you sound just like me some years past.
One of the first things I would do is find a reputable shop that knows engines and has the capabilities to do their own machining. Our original 68 302 did 362hp at 6700 rpm, and that was with a crane blueprint cam purchased new in 1983 and stock heads and manifold. Was everything easy, no! Why, the first time it was rebuilt with different hands they messed things up. Example, had the heads checked and valve job done at a local NAPA that does a lot of machine work. Later when I took the heads to the new engine builder they pulled a vac. check and two seats on one head and one seat on the other leaked. Cause, seats had chatter. The engine machine work done many years ago (engine was never fired) was more upsetting as our original MO was stock bore and did not need bored so it was lightly honed back then yet they used a ridge reamer at the top of the bore. Guess what, they cut in so deep on some cylinders the top land was going to very close so we had to bore .03" over and purchase new pistons. All of this was carefully shown to me by who I now use and swear by. So your discussion brings up memories as I've been there and done that.
That is why I'm suggesting you hold off any purchases and find yourself a reputable shop. Find out exactly what you have, and what you need and why. Ask them to show you and explain. When they rebuilt our 302 I was there for the short-block assembly. When it was broke in and dyno'ed I was there which to me gives credibility to the shop.
Good luck 68Z brother.......