Well, I would first ask if you're making a stiff handle or a bendy handle? A 60" stiff handled bow at those weights and draw length is gonna require very skillful tillering to avoid set. I'd go sinew for sure if that's your plan, but even then it's gonna take good tillering. If you go bendy handle (and I think you should) then it isn't nearly as critical and most any backing would work. I'd recommend rawhide, the thinnest you can find.
Good luck,
George