Need some advice

I have a1964 Dart convertible that has a slant 6 with a 3 speed on the floor. I have been looking to put a 273 V8 in it for a while and came across one with a 904 transmission as a package deal. When I bought the car it came with an extra 4 speed transmission, bell housing and hump for the floor.
I like the thought of an automatic for two reasons easier to drive and the wife would be more likely to drive the car. Part of me likes the 4 speed for nostalgic reasons and probably easier to make work.
Questions
What needs to be done to install the automatic?
Do I go with a floor shift or look for a push button assembly?
Is either better for the V8?
The car was originally a three on the tree.
This is not a high HP motor just a stock 273 non commando.
Thanks
Paul