4.100" bore is not something that many 340/360 blocks will be able to safely handle is the issue with that.......at that point you may as well just pull your pants up and pony up for an R block and be done with it.
Yes, you must sonic check, but many 318s will safely take 4.000" and 340/early 360s will go 4.100" if they pass the test. If you are scared, go 4.030" and live with a 408/416." Aftermarket block is always best, especially for main webbing girth, but at a premium cost/availibility.
