We use a spline for that joint often. Works great.
The top plate is housed 3/4", the 1 1/2" x 5" hardwood spline extends 12"into each plate (from post face). Gable ends have a spline also. During the raising the bents only have to spread the 1 1/2" max for the girt to drop on. Safe, simple, and stronger then short tenons. Sometimes the spline is flush on the bottom, sometimes proud by 3/4" with each end beveled up.