Need some advice

I would not use a push button under any conditions something you want to drive. They are simply too old and no parts. I agree, an aftermarket auto shifter. Something you need to be aware of---all 273's and 318's up to 68? had the so called "small converter bore" in the crank. A later trans with a big converter snout will not fit the 273. I guess there are ways around it but an early converter may not be compatible with some later trans "I believe"

Frankly the car is set up with clutch pedal already, I would stick the 4 speed in. It may not have the 4 speed floor hump you'd have to deal with that