Flat belly, just like you would on a stiff handle, no difference. 1 5/8" is plenty wide enough for a stiff handle. Not sure what split on the belly you refer to, but stiff or bendy, your choice. A bendy should really have no more hand shock than a stiff handle bow. Get your tips narrow and lite. Reflex deflex is one of my favorite designs, but keep it simple this early on IMO (no r/d). If the stave is straight, keep it that way. Dry heat or steam will bend the tips. I use dry heat on dry Osage, but steam will work and is probably safer to bend the tips. A caul is great, but not necessary. If you go bendy handle, 1 1/4 wide for the middle 1/3 and then taper to 3/8" tips. You can narrow the handle a little if you wish, but that's a matter of comfort, you make that call. 64" long. Draw it out and take the sides down to near your profile marks (I leave it a little wide, 1/8" or so on each side). Leave the tips 1" wide for now. start taking wood off the belly until it bends a little at floor tiller. If the handle is the same width as the limb, then the handle will be the same thickness as the limb (near handle), just slightly thicker. Hard for me to say what that thickness will be. Just thin enough to bend a little. If you take a 2x4 and put enough pressure on each end, the board will bend in the middle. Same with your bow, it will bend there first if it is too thin and you don't want that. Get the handle bending last after the limbs are bending some, so leave it thicker to start. I'm no expert, but what you are attempting is a fairly simple design, it will shoot well, it's safe and one I have made plenty of. If you flip the tips, I say be conservative with it at this point.