try this helpful link...http://www.y-blocksforever.com/forums/Topic2815-3-1.aspx#bm2825
Personally, I have used two units from LD Becker of Norwalk, OH, on two recent Y's that I put together. They have an 85 amp alternator, bracketry, pulley, etc. for about $135 I think. That may sound like a lot, but there alternator comes with a LIFETIME warranty. Don't ask me how I know they stand behind their warranty! 
Had to use it one time....btw, the alternator they use is a simple 1 wire GM...
dan