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.
Not a matter of being scared, I already run a .060" 360 block that has held up fine but everything else is aftermarket blocks and yes it comes at a cost but also comes with complete confidence in my blocks holding up too.
