From what you said I read you have a 58 diesel with 6015 J casting code.
First off since the diesel did not start to 59 I would like to see date codes if it is really a 58.
Second since your J block is diesel I guess that blows out the water that J is a gas block.
Now that we got that out the way lets dig deeper.
The EAF 6015 J ... 310609 ... B9NN 6015 J ..... B9NN 6015 B ..... CONN 6015 J are all 172 blocks.
If it is a 172 block it will bolt up and work. Heck the smaller 134 will bolt up and work just has less power.
Some claim the difference in the J and B block is one has sleeves and the other does not.
Others claim the difference is one has the factory balancer.
Some claim the B block is a industrial motor but I have seen tractors with it.
For me I lean to the B9NN 6015 B as being a 1959 block with the factory balancer that was used in late 59 and into 1960. But with so much wrong info being passed around it is hard to tell unless you have personally taken both apart. If I am correct and you now have the B9NN 6015 J block that may be why your block finally came apart. The rod threw because the motor does not have a crank balancer.
The only real difference in the gas and diesel block is the holes for the head bolts and crank bolts.
This could be why both diesel and gas motors can have the same casting numbers.
So I would want to know from the seller of this 6015 B block.
Is this a diesel motor. Pretty obvious question.
And does the motor have a balancer installed. Only way to be sure is the take the pan off.
I would not buy a unbalanced diesel motor as a replacement.
No use asking for more trouble down the road from a unbalanced motor.
I know I did not answer your question but gave you what I know.
Hope it helps as I see you drug up a 2008 post asking me the same question.