An allograft implant for fusing a sacroiliac joint (“SI Joint”), the allograft implant comprising a body having two opposing faces. Each face has one or more anti-migration features, and each face is configured for abutting contact with a sacrum or an ilium of the SI Joint. A graft window is disposed between the two opposing faces, the graft window providing passage through the body between the two opposing faces. The graft window has a cross-sectional area that is about 35% to about 40% of a first cross-sectional area of the body, and about 60% of an area of each opposing face that makes contact with the sacrum or the ilium of the SI Joint.