I've looked for a smaller than stock alternator pulley for the 10SI alternator on my 228 GMC engine for years with no luck. Larger than stock are easy to find. My engine idles at 425 RPM and the 10SI just doesn't have much output at that engine speed. If anyone knows where to find a smaller pulley, please post that info here.

'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.