Slant six transplant ‘64 Barracuda

Easier than slant to V8.
Hunt down the early A slant 6 motor mounts/brackets.
The early A transmission crossmember will work fine with the later 904 trans.
The later oil pan *should* fit fine and have clearance, since you have a drop center V8 centerlink in the car already. The special pan was to provide clearance to the flatter slant 6 centerlink, so you may be okay in that regard.
The throttle linkage on later slants is cable-controlled, and you already have the V8 cable pedal assembly in the car. Will probably need the later, longer slant 6 cable itself. The later kickdown linkage should work in it's entirety.
Use the later (assuming A body) radiator and cooler lines should drop right in with minor tweeking, probably modifying the mounting holes in the radiator brackets or radiator yoke- depending on the year of the rad., may have to trim the brackets down to fit the opening.
You'll have a 7.25 rear end in the '64, nothing else was available until late '65-'66 when the 8.75 was introduced. Use a driveshaft out of a '66 Barracuda/Valiant to connect to your new trans. Use a '66-'67 speedometer cable to hook that up. You'll have to wire in the later trans. backup/NSS to your existing harness.
Then get an aftermarket floor shifter and go cruising.