I think once you start getting near 0.500" of lobe lift, going up to a larger core is a must. If you are paying for a custom core, plus enlarging the tunnel, it wouldn't make sense to not go up to 60 mm. There is a conversion babbit bearing that allows the use of a 65 mm core in a roller 60 mm tunnel as well. Or go up again using clamshell bearings, which gets spendy.