I'd build a jig that holds the workpieces in position for whatever tool you choose to make the cut. The shape will have to match the tool. If you have a router table, and the cut is not too large, you can use a square cut piece of ply or MDF to push each 2x4 piece across the bit. The MDF will hold the piece square and avoid chip out.
If you cut with a circular saw, you'll need to work out a flat top plate with a built in fence for the saw to ride on, and that gets a solid grip on the 2x. Push the saw across to make the cut, and let the piece that straddles the 2x as a backer to limit tearout. With most circular saws, you're probably going to have to use a 45 degree blade tilt.
You'll also need a small stop block against which you'll push each block so all pieces have matching cuts. I'd make the table wide and long enough to keep the well saw supported throughout the entire cut. Add a couple of strips of low friction tape to make the saw behave itself.
If you have a table saw, a long piece attached to the miter gauge, coated with some self adhesive sand paper, and with a stop block for the back end will do. Probably the safest option.
I also agree that using a long bolt will probably split at least several pieces, depending on their grain and density. Of course, you could drill a 5/8ths hole and insert a hardwood dowel, then put a shorter screw through the end, into the dowel. This will distribute the load through the length of the dowel and reduce the chance of splitting.
Making the suggested circular saw jig will take maybe 90 minutes to complete.
The more I do, the less I accomplish.