After a really quick search (http://mustangtek.com/4300/4300.html) , the closest thing to your number is C8SF-G or C8SF-H which are for 68 429's. Any C8** with an A designator seem to be for smaller engines. You may have been slightly off with the C857A number---check a little closer.

I was wrong in my previous post, the larger 4300 had a 1.25" bore while the smaller one had 1.00" bores.

I did a Google on Autolite 4300 and saw several sites, just go to all of them. LOL