Golden Pheasant are very stout loads. I would not put your fine old Greifelt to that much abuse. Just buy low pressure loads in a 2 1/2" case, available from lots of sources, including PolyWad.com. These are not wimpy loads just because they are low pressure, but they are designed for older guns. There is no reason to lengthen the chambers. Your gun is likely proofed for only a 1 oz load, not 1 1/8, and definitely not the high pressure Golden Pheasant.