Could just butt it against the pine, I doubt most people would object.
Alternative, cut the pine boards in a straight vertical line and inset your filler piece in the gap (pine boards now but against your filler strip). Downside is you're likely to have some splintering and pretty tough to keep cut line as straight as your filler strip.
Alternative, cut the pine boards in a straight vertical line and inset your filler piece in the gap (pine boards now but against your filler strip). Downside is you're likely to have some splintering and pretty tough to keep cut line as straight as your filler strip.