By looking at the thread beanhead has shown above, it won't take long to determine the transmission. Personally, I don't believe that Chrysler ever offered a 904 behind a big block. From 1962 to 1966 the 727 converter was a 12", narrow ring gear, medium stall converter, with 19 splines. From 1967 on, all 727's have 24 splines.
904's from 1962 had 18 splines, and from 1968 on they had 27 splines.
When it comes time to order your high stall, go direct to a high stall builder. Dynamic, Precision and PTC are mentioned as being the places to go. Here's my thinking behind this. While Jegs, Summit and the like are great places for some items, they lack the personal with real performance converter knowledge. A converter builder will ask you all kinds of questions about your engine, tire size, diff ratio etc., and from that information they will build you a converter best suited for your particular situation. Having worked in the industry, I know that if you don't give them honest answers to their questions, your converter won't perform properly. If you check their websites, most if not all will have a specification sheet for you to look at and see what they will be asking you.
The only other advise I can pass along: Don't let price be your only guide, as you get what you pay for.
.
