Dennis Piranio has a very good conversion kit(and other stuff) http://www.modelaparts.net/webbcat.htm/catp31.htm Worked for me. His page is kinda hard to get around in, but lots of good stuff and info.
My understanding is that these are the parts that Snyder's sells.
Additionally, Richard offers oversize intake values (requiring also oversize valve seats, mandating minor block machining), which do not seem to be listed in Snyder's or others' catalog listings.
I intend no derogation towards Piranio's offering, I just have no experience or knowledge of them.