Depends on the pickup. A 4.25 stroke with 2.20 journal clears most pickups but not all. A 4.15 with 2.375 journal is slightly larger than a 4.25/2.20 so you'll have even less room. If you haven't already drilled and tapped the block for 1/2 inch pickup you might consider just staying with 3/8. Or at least mock it up before you decide.