I'm pretty sure I've read a post on converting the stock ammeter in our trucks to be a voltmeter, but I haven't been able to find the post. Would someone with better search skills than I please do it for me?

I'm looking for a friend who wants a voltmeter in the stock gauge cluster. It probably involves ripping out the ammeter guts and replacing it with voltmeter guts, but what parts are used?

'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.