Vintage air has a neat looking system designed for these trucks.
http://www.vintageair.com/download/pdf_singles/45.pdf It is what I've been eyeing for my 50. More so now that we have been hitting 100 quite a bit. Most of the 12v generators that came on chevys should bolt up. Check with Fifth Avenue at the links. They have everything you need to do a 12v conversion and they are nice too. I think they moderate the electrical board?. I'm converting to an alternator as my commutes are short and slow and generaotrs need the revs to put out.