Eric,
Have you considered using a router sled? If you're not averse to power tools, I think this is ideal for a split top bench.
I flattened the Scandinavian workbench I previously showed by hand, but for this one I used the planer sled. It is 3x7 bowling alley w/ DF aprons and white oak front dog hole apron.
Bench.jpg
I started doing it by hand, but quickly realized how time consuming the task was going to be, not to mention my shoulders and back aching.
I used a Magnate 2" planer bit. Just a little bit of sanding took the tracks out.
Here is the set up. Once I get it set up it took me less than 30 minutes to do the whole top.
Flattening sled.jpg
BTW, we are all anxiously waiting for some pics!!