I went back and looked at Aldavs link on those straight bits.
Everyone wants the latest multi-insert carbide wonder bit, costing a fortune.
The 1/2" shanked 3/4" wide will do fine. Use the bigger 1.25" if you are feeling extra brave, but I really dont think one needs to go to huge expense to solve this problem.
Its a slab. You want to get it flat with a jig. It doenst need to be finished perfectly AND a little bit of extra time wont kill you.
I'd choose to economise on this one and use a 3/4" wide bit from the multi-bit el-cheapo box.
Attachment 474553