4.88 would be a "safe" start.
I'm a small block guy and have seen many
auto trans 3.58 stroke 360's favoring a 4.88 while the 3.31 stroke 340's like a 5.13 in Stock. Your 383 has a 3.375 stroke, so 4.88 should be safe and later determine if the engine and valvetrain can handle a 5.13. There are always 92.5" and 94" circumference slicks to experiment with as well as radials which will raise the rpms due to limited growth. But it wouldn't surprise me if a couple combos may go a step further. That's my theory and 2 cents worth. Now I'm curious if someone can confirm my theory.
See if you can find any Moser dealers in you area. Might get a better deal than buying direct. I lucked out to have a driveline place here in Daytona who also do my driveshafts. Or ask about "price matching"!