Without buying or borrowing a slotting tool such as this
http://www.timbertools.com/Products/SwissPRO-Horizontal-Slotter-SG-400.html (and it only goes 18" in) or plunging it with a chainsaw (not recommended as it takes a perrfectly tuned saw , tremendous skill and years of heavy chainsaw use to be able to do that accurately) you'll have to do as Ray said and cut an open slot down the face of the post and then fill it in after the post is installed. Depending on your tooling and the orientation of the slot in the 6x8, you may need to cut the slot from both sides of the post and then fill both faces. Check with the engineer to verify that this is ok.
