Is your Valiant a column shift or console? It's a cable shifted tranny, so if you want to keep your factory console you will want to stick with a 1965 cable shifted V8 A904. If you switch to linkage shifted, you'll have to change other stuff...that's all.
I plan for a 340 into my '65 Barracuda and I'll keep my cable shifted 904 but have it gone over by a performance tranny shop I know.