The belt should never contact the bottom of the sheave. If it does, the sides of the belt are not doing their job and the belt will slip (and hopefully make noise so you will know something is wrong).
Try feeding something small like a piece of solid (not stranded) 18 gauge wire or maybe a flattened soda straw between the bottom of the belt and the pulley. If it will slide between them the belt is not riding on the bottom of the sheave (that is good). If you can't, I'd recommend get a one step wider belt.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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