rebuilding cab over bunk - plywood base construction

I have to disagree. Scarf joints we invented to make boards longer, not wider. The seam is stapled together but more as a clamp until the adhesive set and not as holding method. If you're concerned about it use a biscuit joiner or install a spline. If you wanted you could use electrical staples but it's really not necessary.

Baltic birch ply comes in 5' widths but you would be just moving the joint from horizontal to perpendicular.

DOH :S - just use 3/4" T&G (tongue & groove) Subfloor plywood.