This is the final size of the back, after I milled a 5/8" piece off the stock. I used a 5" slotting blade that was about 1/16" wide. I would have used a thinner blade but I didn't have one in a 5" or 6" size. A 4" wouldn't give me enough clearance.
So, you always wanted to build handsaws...
Making a Handle
Assembling the Saw