This suggestion by Chris is the same as I made at the start (I recommended chalk to mark the high spots). Use a short plane to concentrate on the high spots. I differ in process in that the edges should continue to be match planed since it is difficult to control for squareness over such long boards. This leaves you free to focus on just one dimension.Assuming it’s straight, I’d then look at the seam and look for gaps. I would mark all the spots where the boards touched and not where there were gaps (gaps were usually at both ends or in the middle). Then I’d take board #1 off, grab a smoother or jack, and just hit the spots I marked.
Just the high spots. Short plane. Remember that the match planing doubles the shaving thickness.
Regards from Perth
Derek