You've got a lot to change for a 340 stroker, not one 904 built w/a pushbutton will handle that engine w/o upgrades to later than '68 components. And I don't believe there were
any SB-727 case PB trannies either, so you need a trans shop to take a 273 PB trans & perform all the upgrades to handle that stroker, or ditch the pushbuttons. There were a
couple companies that made PB cable adapting hardware to std. trans', Imperial(services?) I believe is one, google them.
Also, there isn't anything smaller than the 904 in your '64, the only "smaller" 904
was the Masterbishi adapted versions for 4cyls.