Possible alternative tops
2 alternatives to making your own bench top are:
Ikea (ya I know bad four letter word!) sells laminated wood counter tops in various lengths. You even get an 8 footer cut it down to two four foot planks. The bottom one could then be cut into three sections forming the dados to sit over your saw horses like James mentioned.
Another way is to look locally for what is call Ganglam. These are BIG architectural laminated beams used to span large rooms. Many times construction companies have off cuts left over you can pickup/purchase.