Use 18 2x4s for the top substrate -- 57". Glue the 2x4s in sets of five. After all the sets have cured, glue them together to create the full top substrate assembly. Face-glue and clamp the 2x4s for the top substrate so they give the appearance of a butcher-block top. The substrate gives the bench strength and stability.
Use a belt sander or hand plane to flatten the top of the substrate. Lay the substrate out, top side down. Align two short 2x8 top rails -- 27" -- flush with the short sides of the substrate and use 3" wood screws to secure the rails to the substrate.
Align two long 2x8 top rails -- 60" -- with the long sides of the substrate. The ends of the long top rails should be flush with the outside face of each short top rail. Secure the long top rails to the substrate with 3" wood screws.
Insert two 2x4 rail stretchers -- 27" -- between the long rails, positioned 1/3 of the way in from each end of the bench. Drive 3" wood screws through the long rails into the ends of the rail stretchers.
Construct four leg assemblies using eight 2x4 leg pieces -- 30". Butt the edge of one 2x4 leg piece to the face of another and line the ends up flush. Screw the leg pieces together with 3" wood screws.
Set the legs inside the corners where the long and short top rails meet. The joints between the leg assemblies should be visible from the short sides of the bench. Bore 3/8" holes through the top rails and legs. Insert 3/8" x 4" carriage bolts from the outside face of the top rails through the leg pieces. Install a washer and nut on the backside of each carriage bolt and tighten.
Measure 12" from the bottom of each leg and mark. Align the tops of the two long 2x4 bottom rails -- 54" -- flush with the marks and use 3" wood screws to secure the rails to the legs.
Insert the two short 2x4 bottom rails -- 21" -- between the long 2x4 bottom rails. The outside face of the bottom rail should butt to the inside face of the leg. Secure the short rails to the legs with 3" wood screws.
Stand the workbench right side up. The bench will be heavy, so have someone help you. Align the edges of the 1/2" plywood top flush with the edges of the bench. Secure the top to the substrate with 1" flathead screws. If the plywood top is damaged later, it's a simple task to remove and replace it.