Guys I have been full circle on this alternator thing, and here's what I have done for alternators for my P400 build -
Because I am fully dressing the P400 without taking anything off my running 6.2, I purchased a couple of used OEM alts from a vendor on this site with the intention of rebuilding them myself. Well they came in, and they were awfully nasty, so I immediately opened the case of the first one to inspect. The inside was toast. The rotor looked like it had been welded and the stator was rusted beyond recognition. No way I was going to fool with these things for my P400 engine build. So I began to explore online for rebuilt Delco 27Si units. Most sources for rebuilt units were using cheap Chinese stuff, and I did not want any part of that. But then I discovered Wilson in Daphne, Alabama. Wilson does not just "rebuild" starters and alternators, they "remanufacture" them with all new rotors and stators and internal parts. I called Wilson and spoke with a fellow named Phil in technical service, and he stated that they have remanufactured Delco 27Si units in stock and sell through NAPA and other outlets. The part number for the Delco 27Si isolated ground unit was 90-01-4277. I called NAPA, and they priced out a pair for $203.56 each with core exchange. I ordered 2 and was happy to exchange the boat anchors. I picked them up today. The Wilson units are top notch and each came with a document showing the test data. My units each tested to produce 115 amps.
http://www.wilsonautoelectric.com/
The Wilson units came with no fan or pulley, and I destroyed the original pulleys trying to get them off the boat anchors. So I needed new fans and pulleys. So I purchased brand new USA-made fans and pulleys from Summit, manufactured by Tuff Stuff Performance in Cleveland, Ohio
https://www.tuffstuffperformance.com/ . They fit perfectly and the zinc cad plating adds a nice touch. Tuff Stuff item #7600AD. $23.99 each on Summit.
Hope this helps. By the way, I have 2 complete rebuild kits available if anybody is interested.