273 and push button

The 904 from the '64 slant will not fit the 273.
Imperial services used to offer a kit to make the later 904s work with the pushbutton shift mechanism, but their availability has been pretty sketchy the past few years.
You will need a different driveshaft for the '69 904, your '64 has a ball and trunion driveshaft and the '69 transmission uses a slip yoke.
Your slant 6 car uses a different throttle pedal and linkage than a V8.
There COULD be clearance issues with the '69 kickdown linkage when used in the '64.
You didn't say if your '64 is a Barracuda/Valiant or a Dart.
Easiest solution:
Get a steering column from a '66 A body, complete with shift linkage and swap it in. They come in both power and manual steering lengths, get the one you need. This will allow you to shift the '69 transmission.
Get a 904 driveshaft from a '66 Barracuda/Valiant or Dart (they are 2 different lengths)- whichever one you need. This will allow you to use the '69 transmission.
Get the throttle pedal/ linkage and kickdown linkage from a V8 early A body.
Locate some V8 engine mount brackets from an early A body (early A is '66 and earlier, by the way).
Get a V8 steering center link. They have a larger center drop for V8 oil pan clearance.
You will also need the early V8 radiator (the '69 one can work).
Seriously consider upgrading your brakes/torsion bars.
On second thought, the easiest solution is:
Freshen up your current slant 6/904 combination. They're not so bad!