Flat Square 8"x 8"x 10' Old Growth Timbers

Not certain if this is the best way as I have never done this but I would try loosely clamping them together on 4 saw horses. Only using enough pressure to keep them beside each other and run a saw down the joint, the clamps really would only exist to keep the boards from coming apart as you cut so the saw doesn’t skip over points. It may be a good idea to have some splines the width of the saw kerf to insert as you move along so the clamps can be moved without actually clamping the boards together where you have already cut. I would suspect this will still require cutting the joint at least a couple times to get it set really close together. If the beams are badly warped or twisted the whole idea probably wouldn’t work that well. It obviously requires that you use a handsaw.

-- A posse ad esse

8×8??? Pine? would you consider cutting them in half? otherwise I think threaded rod/washers/nuts are in order.

You didn’t mention having a jointing plane, barring the purchase of one,I would suggest a squaring cradle.

Out of 3/4” ply wood build a cradle with a bottom, 2 end caps, and two parallel side runners at the top. It should be set just below what you need to flatten the log, and just wide enough the fit the piece. shim the board steady, and screw it to the end caps. At 8” you should be able to start working the face at a skew, so that the foot of the plane is always over side runners. Plane the material down until the plane is riding on the side runners. This works best with longer planes than what you describe. You can create a “shoe” that the plane rides in. just ensure the base of the plane is below the shoe and plane until the shoe rides on the side runners. The side runners should be tall enough that they don’t flex, but have an opening to shim under the board to the base. After you joint the face this way, you can turn the board 90 degress and carefully square you joined face to the side rail, and joint that face. If you really must do it by hand, that would work.

In my shop I would just throw it into a planing sled and run it through my power planer, as I only have a 6” jointer.

-- Who is John Galt?

Id go for the threaded rod all the way through them to cinch them nice n tight once i was happy with the joints id make a large router sled to plane the top flat. Wrestling aorund 8” x 8” beams doesnt sound like much fun. With the rod through you should be able, theoretically, to loosen and tighten them up with changes in season.

I cant wait to see it.

-- Its not a crack, its a casting imperfection.