If you were shooting for a 40# bow like you first mentioned and were going with the ELB design you might want to start with an inch wide at the grip and a straight taper to about 3/8th inch tips. You might even end up narrowing the handle down to 7/8"!
With 72" of bow, even if you had a very stiff handle area you have more than enough working limb to accommodate that kind of arc. If you are uncomfortable with a bare back or you have any concerns about grain issues a little antelope rawhide backing, or even brown paper bag strips would resolve that issue.
I've turned out a few unbacked hickory ELB's in the 60# range. Most of them took about 2" of set, so I think I was going too narrow at the 1" handle, consequently to get the weight I had to make them to thick front to back. I wasn't following the 5/8 rule, just scraping away. (divide your width by 8, multiply by 5 to get your limb depth, ex: inch wide = 5/8 inch deep).