You need to do some research on bolt grades. Rather than me trying to post that information, it is easy to fine with a Google Search.
https://www.boltdepot.com/fastener-info ... chart.aspx The one and only thing that makes a fastener hold your head tight to the block is first the torque on the stud (bolt) The second and equally important part of that equation is the ability of the fastener to stretch and return mostly to the original length. The cast iron in an Old IHC engine block is not strong enough to hold a grade 8 bolt while it is being stretched (tightened).
Grade 8 is not a size, but refers to the strength of the fastener and its ability to resist stretching. Caterpillar make their own fasteners and the grades go much higher than grade 8. The sever duty that a CAT may have endure will overwhelm grade 8 in a New York Minute.
What is thought to be beter may not be the best solution for a situation. My friend was having trouble with getting the heads on a Flathead V8 in a 1932 Ford Pickup to seal. He called me of "any idea:. They were using grade 8 long set-screws and grade 8 nuts against grade 8 washers. They had exceeded the maximum torque by 50% and were at risk of pulling the threads out of the block.
Something else you must consider as a better than real; possibility is, the Old IHC castiron will have deteriorated from thousands of heat cycles and the threads in the block will have weakened considerably. For this reason,
you should strongly consider studs rather than bolts. Studs can be installed while not under tightening stress and a custom stud can go deeper than a random bolt. the bolt must move in the old threads while it is being tightened. This is double stress on the block threads. Ask ARP about thread sealant for the block to stud connection. Some threaded holes may go into the water jacket and improper sealing may (read that is will) cause a leak. This old engine most likely does not have a pressurised cooling system, so the leaking possibility will be much less.
If you are planning to change the cooling system to pressurized, that is much more to consider than plugging off the overflow tube and adding a pressure cap.
Do not run an inexpensive thread tap into the Old IHC block threads because the tap may remove metal, and this is not what you want to do. There are thread reconditioning taps that can move and improve the block threads. Back in the day cast iron could have an amount of sulfur in the mix. While this made the castiron flow better and leave fewer inclusions, the strength after 60- 70 years may be reduced.
https://arp-bolts.com/kits/product.php?PL=22 This company makes the best fasteners for you situation. ARP may also sell the thread conditioning taps. The little extra expense of ARP is nothing in comparison to the cost of repairing a screw-up.