Federal NyClad 125gr for a non +P load. Winchester used to produce a 158gr lead SWC at standard pressure which I would not hesitate to use; dunno if its still available.
Just curious... if you're contemplating belt carry, why stray from the G26? J frames are meant for pockets. if you're going to wear a holster, might as well fill it with something a bit more substantial, but just as easily concealable.
Measure the thickness of your G26 slide, then do the same on the 637's cylinder...