My last work bench was a big project.
The top is 2 x 4s on edge doweled together with 8 3/4" dowels 2 x6s in to form the pockets for the legs. bench top is 30" x 8' x 3 1/2" thick except where the 2 x 8s are.
The legs are 3 2 x 6s doweled together with a slot left for the cross support to go through. The top center boards bottom edge is cut at a 10* angle. This slot is roughly 4 1/2" wide. A wedge was made with the matching 10* angle on top side.
The legs are also notched for the cross braces front to back.
Legs were inserted into their pockets and 1 dowel set thru the top. then the long leg braces installed the legs squared and the braces glued then the wedges drove in tight when dry they were glued and doweled. The front to back leg braces were inserted in their pockets and glued and doweled.
The tops dowels were set below center so the top can be planed down several times to clean it up as needed.
This bench is heavy it dosnt wiggle shake or move. there are no nails screws or bolts. other than the bolts that hold the vises on. its solid and stable.
The bottom leg braces and shelf are at a hieght I can use my die cart to move it when needed.
I really recommend some 2x4s or other on edge even using a solid door. A simple square of 2 x 4s on edge with one down the center or in a X pattern really stiffen the top and make a good mounting point for the legs. Use 6 x 6s or 3 2x6s laminated and doweled together. leg braces front to back and length wise to stiffen support the legs. A cople cross bars in this and some thin plywood makes a shelf and the flat plywood really stiffens the base up.
When building a bench use plenty of glue in the joints and lock the joints together in a solid pattern.