Schumacher headers (www.engine-swaps.com). They call them Tri-Y and I'm running them on my 71 Demon with a 1970 383 and 727. They don't capture the starter and you can actually change the spark plugs.
TTI has a couple Big block A-body headers. Higher priced than avarage, but many will agree that they are some of the nicest, best fitting headers around. Lot's more info on their web site: